CHAPTER XVI

Fig. 10. Climatic changes of 140,000 years as inferred from the stars.According to the electro-stellar hypothesis, Alpha Centauri is more important climatically than any other star in the heavens not only because it is triple and bright, but because it is the nearest of all stars, and moves fairly rapidly. Sirius and Procyon move slowly in respect to the sun, only about eleven and eight kilometers per second respectively, and their distances at minimum arefairly large, that is, 8 and 10.2 light years. Hence their effect on the sun changes slowly. Altair moves faster, about twenty-six kilometers per second, and its minimum distance is 6.4 light years, so that its effect changes fairly rapidly. Alpha Centauri moves about twenty-four kilometers per second, and its minimum distance is only 3.2 light years. Hence its effect changes very rapidly, the change in its apparent luminosity as seen from the sun amounting at maximum to about 30 per cent in 10,000 years against 14 per cent for Altair, 4 for Sirius, and 2 for Procyon. The vast majority of the stars change so much more slowly than even Procyon that their effect is almost uniform. All the stars at a distance of more than perhaps twenty or thirty light years may be regarded as sending to the sun a practically unchanging amount of radiation. It is the bright stars within this limit which are important, and their importance increases with their proximity, their speed of motion, and the brightness and number of their companions. Hence Alpha Centauri causes the main maximum in Fig. 10, while Sirius, Altair, and Procyon combine to cause a general rise of the curve from the past to the future.Let us now interpret Fig. 10 geologically. The low position of the curve fifty to seventy thousand years ago suggests a mild inter-glacial climate distinctly less severe than that of the present. Geologists say that such was the case. The curve suggests a glacial epoch culminating about 28,000 years ago. The best authorities put the climax of the last glacial epoch between twenty-five and thirty thousand years ago. The curve shows an amelioration of climate since that time, although it suggests that there is still considerable severity. The retreat of the ice from North America and Europe, and its persistence in Greenland and Antarctica agree with this. And the curveindicates that the change of climate is still persisting, a conclusion in harmony with the evidence as to historic changes.If Alpha Centauri is really so important, the effect of its variations, provided it has any, ought perhaps to be evident in the sun. The activity of the star's atmosphere presumably varies, for the orbits of the two components have an eccentricity of 0.51. Hence during their period of revolution, 81.2 years, the distance between them ranges from 1,100,000,000 to 3,300,000,000 miles. They were at a minimum distance in 1388, 1459, 1550, 1631, 1713, 1794, 1875, and will be again in 1956. In Fig. 11, showing sunspot variations, it is noticeable that the years 1794 and 1875 come just at the ends of periods of unusual solar activity, as indicated by the heavy horizontal line. A similar period of great activity seems to have begun about 1914. If its duration equals the average of its two predecessors, it will end about 1950. Back in the fourteenth century a period of excessive solar activity, which has already been described, culminated from 1370 to 1385, or just before the two parts of Alpha Centauri were at a minimum distance. Thus in three and perhaps four cases the sun has been unusually active during a time when the two parts of the star were most rapidly approaching each other and when their atmospheres were presumably most disturbed and their electrical emanations strongest.The fact that Alpha Centauri, the star which would be expected most strongly to influence the sun, and hence the earth, was nearest the sun at the climax of the last glacial epoch, and that today the solar atmosphere is most active when the star is presumably most disturbed may be of no significance. It is given for what it is worth. Its importance lies not in the fact that it proves anything, but that no contradiction is found when we test the electro-stellar hypothesis by facts which were not thought of when the hypothesis was framed. A vast amount of astronomical work is still needed before the matter can be brought to any definite conclusion. In case the hypothesis stands firm, it may be possible to use the stars as a help in determining the exact chronology of the later part of geological times. If the hypothesis is disproved, it will merely leave the question of solar variations where it is today. It will not influence the main conclusions of this book as to the causes and nature of climatic changes. Its value lies in the fact that it calls attention to new lines of research.Fig. 11Fig. 11. Sunspot curve showing cycles, 1750 to 1920.Note.The asterisks indicate two absolute minima of sunspots in 1810 and 1913, and the middle years (1780 and 1854) of two periods when the sunspot maxima never fell below 95. If Alpha Centauri has an effect on the sun's atmosphere, the end of another such period would be expected not far from 1957.CHAPTER XVITHE EARTH'S CRUST AND THE SUNAlthough the problems of this book may lead far afield, they ultimately bring us back to the earth and to the present. Several times in the preceding pages there has been mention of the fact that periods of extreme climatic fluctuations are closely associated with great movements of the earth's crust whereby mountains are uplifted and continents upheaved. In attempting to explain this association the general tendency has been to look largely at the past instead of the present. Hence it has been almost impossible to choose among three possibilities, all beset with difficulties. First, the movements of the crust may have caused the climatic fluctuations; second, climatic changes may cause crustal movements; and third, variations in solar activity or in some other outside agency may give rise to both types of terrestrial phenomena.The idea that movements of the earth's crust are the main cause of geological changes of climate is becoming increasingly untenable as the complexity and rapidity of climatic changes become more clear, especially during post-glacial times. It implies that the earth's surface moves up and down with a speed and facility which appear to be out of the question. If volcanic activity be invoked the problem becomes no clearer. Even if volcanic dust should fill the air frequently and completely, neither its presence nor absence would produce such peculiar featuresas the localization of glaciers, the distribution of loess, and the mild climate of most parts of geological time. Nevertheless, because of the great difficulties presented by the other two possibilities many geologists still hold that directly or indirectly the greater climatic changes have been mainly due to movements of the earth's crust and to the reaction of the crustal movements on the atmosphere.The possibility that climatic changes are in themselves a cause of movements of the earth's crust seems so improbable that no one appears to have investigated it with any seriousness. Nevertheless, it is worth while to raise the question whether climatic extremes may coöperate with other agencies in setting the time when the earth's crust shall be deformed.As to the third possibility, it is perfectly logical to ascribe both climatic changes and crustal deformation to some outside agency, solar or otherwise, but hitherto there has been so little evidence on this point that such an ascription has merely begged the question. If heavenly bodies should approach the earth closely enough so that their gravitational stresses caused crustal deformation, all life would presumably be destroyed. As to the sun, there has hitherto been no conclusive evidence that it is related to crustal movements, although various writers have made suggestions along this line. In this chapter we shall carry these suggestions further and shall see that they are at least worthy of study.As a preliminary to this study it may be well to note that the coincidence between movements of the earth's crust and climatic changes is not so absolute as is sometimes supposed. For example, the profound crustal changes at the end of the Mesozoic were not accompanied by widespread glaciation so far as is yet known, althoughthe temperature appears to have been lowered. Nor was the violent volcanic and diastrophic activity in the Miocene associated with extreme climates. Indeed, there appears to have been little contrast from zone to zone, for figs, bread fruit trees, tree ferns, and other plants of low latitudes grew in Greenland. Nevertheless, both at the end of the Mesozoic and in the Miocene the climate may possibly have been severe for a time, although the record is lost. On the other hand, Kirk's recent discovery of glacial till in Alaska between beds carrying an undoubted Middle Silurian fauna indicates glaciation at a time when there was little movement of the crust so far as yet appears.[129]Thus we conclude that while climatic changes and crustal movements usually occur together, they may occur separately.According to the solar-cyclonic hypothesis such a condition is to be expected. If the sun were especially active when the terrestrial conditions prohibited glaciation, changes of climate would still occur, but they would be milder than under other circumstances, and would leave little record in the rocks. Or there might be glaciation in high latitudes, such as that of southern Alaska in the Middle Silurian, and none elsewhere. On the other hand, when the sun was so inactive that no great storminess occurred, the upheaval of continents and the building of mountains might go on without the formation of ice sheets, as apparently happened at the end of the Mesozoic. The lack of absolute coincidence between glaciation and periods of widespread emergence of the lands is evident even today, for there is no reason to suppose that the lands are notably lower or less extensive now than they were during the Pleistocene glaciation. In fact, there is much evidence that many areas have risensince that time. Yet glaciation is now far less extensive than in the Pleistocene. Any attempt to explain this difference on the basis of terrestrial changes is extremely difficult, for the shape and altitude of continents and mountains have not changed much in twenty or thirty thousand years. Yet the present moderately mild epoch, like the puzzling inter-glacial epochs of earlier times, is easily explicable on the assumption that the sun's atmosphere may sometimes vary in harmony with crustal activity, but does not necessarily do so at all times.Turning now to the main problem of how climatic changes may be connected with movements of the earth's crust, let us follow our usual method and examine what is happening today. Let us first inquire whether earthquakes, which are one of the chief evidences that crustal movements are actually taking place in our own times, show any connection with sunspots. In order to test this, we have comparedMilne's Catalogue of Destructive Earthquakesfrom 1800 to 1899, with Wolf's sunspot numbers for the same period month by month. The earthquake catalogue, as its compiler describes it, "is an attempt to give a list of earthquakes which have announced changes of geological importance in the earth's crust; movements which have probably resulted in the creation or the extension of a line of fault, the vibrations accompanying which could, with proper instruments, have been recorded over a continent or the whole surface of our world. Small earthquakes have been excluded, while the number of large earthquakes both for ancient and modern times has been extended. As an illustration of exclusion, I may mention that between 1800 and 1808, which are years taken at random, I find in Mallet's catalogue 407 entries. Only thirty-seven of these, which were accompanied by structural damage, have been retained.Other catalogues such as those of Perry and Fuchs have been treated similarly."[130]If the earthquakes in such a carefully selected list bear a distinct relation to sunspots, it is at least possible and perhaps probable that a similar relation may exist between solar activity and geological changes in the earth's crust. The result of the comparison of earthquakes and sunspots is shown in Table 7. The first column gives the sunspot numbers; the second, the number of months that had the respective spot numbers during the century from 1800 to 1899. Column C shows the total number of earthquakes during the months having any particular degree of spottedness; while D, which is the significant column, gives the average number of destructive earthquakes per month under each of the six conditions of solar spottedness.TABLE 7DESTRUCTIVE EARTHQUAKES FROM 1800 TO 1899 COMPARED WITH SUNSPOTSABCDEFSunspot numbersNumber of months per Wolf's TableNumber of earthquakesAverage number of earthquakes per monthNumber of earthquakes in succeeding monthAverage number of earthquakes in succeeding month0-153445221.525121.4915-301943061.583101.6030-502374331.834391.8550-701954022.063902.0070-1001352862.123102.30over 100952182.301751.84The regularity of column D is so great as to make it almost certain that we are here dealing with a real relationship. Column F, which shows the average number of earthquakes in the month succeeding any given condition of the sun, is still more regular except for the last entry.The chance that six numbers taken at random will arrange themselves in any given order is one in 720. In other words, there is one chance in 720 that the regularity of column D is accidental. But column F is as regular as column D except for the last entry. If columns D and E were independent there would be one chance in about 500,000 that the six numbers in both columns would fall in the same order, and one chance in 14,400 that five numbers in each would fall in the same order. But the two columns are somewhat related, for although the after-shocks of a great earthquake are never included in Milne's table, a world-shaking earthquake in one region during a given month probably creates conditions that favor similar earthquakes elsewhere during the next month. Hence the probability that we are dealing with a purely accidental arrangement in Table 7 is less than one in 14,400 and greater than one in 500,000. It may be one in 20,000 or 100,000. In any event it is so slight that there is high probability that directly or indirectly sunspots and earthquakes are somehow connected.In ascertaining the relation between sunspots and earthquakes it would be well if we could employ the strict method of correlation coefficients. This, however, is impossible for the entire century, for the record is by no means homogeneous. The earlier decades are represented by only about one-fourth as many earthquakes as the later ones, a condition which is presumably due to lack of information. This makes no difference with the methodemployed in Table 7, since years with many and few sunspots are distributed almost equally throughout the entire nineteenth century, but it renders the method of correlation coefficients inapplicable. During the period from 1850 onward the record is much more nearly homogeneous, though not completely so. Even in these later decades, however, allowance must be made for the fact that there are more earthquakes in winter than in summer, the average number per month for the fifty years being as follows:Jan. 2.8May 2.4Sept. 2.5Feb. 2.4June 2.3Oct. 2.6Mar. 2.5July 2.4Nov. 2.7Apr. 2.4Aug. 2.4Dec. 2.8The correlation coefficient between the departures from these monthly averages and the corresponding departures from the monthly averages of the sunspots for the same period, 1850-1899, are as follows:Sunspots and earthquakes of same month: +0.042, or 1.5 times the probable error.Sunspots of a given month and earthquakes of that month and the next: +0.084, or 3.1 times the probable error.Sunspots of three consecutive months and earthquakes of three consecutive months allowing a lag of one month, i.e., sunspots of January, February, and March compared with earthquakes of February, March, and April; sunspots of February, March, and April with earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the probable error.These coefficients are all small, but the number of individual cases, 600 months, is so large that the probable error is greatly reduced, being only ±0.027 or ±0.028. Moreover, the nature of our data is such that even ifthere is a strong connection between solar changes and earth movements, we should not expect a large correlation coefficient. In the first place, as already mentioned, the earthquake data are not strictly homogeneous. Second, an average of about two and one-half strong earthquakes per month is at best only a most imperfect indication of the actual movement of the earth's crust. Third, the sunspots are only a partial and imperfect measure of the activity of the sun's atmosphere. Fourth, the relation between solar activity and earthquakes is almost certainly indirect. In view of all these conditions, the regularity of Table 7 and the fact that the most important correlation coefficient rises to more than four times the probable error makes it almost certain that the solar and terrestrial phenomena are really connected.We are now confronted by the perplexing question of how this connection can take place. Thus far only three possibilities present themselves, and each is open to objections. The chief agencies concerned in these three possibilities are heat, electricity, and atmospheric pressure. Heat may be dismissed very briefly. We have seen that the earth's surface becomes relatively cool when the sun is active. Theoretically even the slightest change in the temperature of the earth's surface must influence the thermal gradient far into the interior and hence cause a change of volume which might cause movements of the crust. Practically the heat of the surface ceases to be of appreciable importance at a depth of perhaps twenty feet, and even at that depth it does not act quickly enough to cause the relatively prompt response which seems to be characteristic of earthquakes in respect to the sun.The second possibility is based on the relationship between solar and terrestrial electricity. When the sun is active the earth's atmospheric electrical potential issubject to slight variations. It is well known that when two opposing points of an ionized solution are oppositely charged electrically, a current passes through the liquid and sets up electrolysis whereby there is a segregation of materials, and a consequent change in the volume of the parts near the respective electrical poles. The same process takes place, although less freely, in a hot mass such as forms the interior of the earth. The question arises whether internal electrical currents may not pass between the two oppositely charged poles of the earth, or even between the great continental masses and the regions of heavier rock which underlie the oceans. Could this lead to electrolysis, hence to differentiation in volume, and thus to movements of the earth's crust? Could the results vary in harmony with the sun? Bowie[131]has shown that numerous measurements of the strength and direction of the earth's gravitative pull are explicable only on the assumption that the upheaval of a continent or a mountain range is due in part not merely to pressure, or even to flowage of the rocks beneath the crust, but also to an actual change in volume whereby the rocks beneath the continent attain relatively great volume and those under the oceans a small volume in proportion to their weight. The query arises whether this change of volume may be related to electrical currents at some depth below the earth's surface.The objections to this hypothesis are numerous. First, there is little evidence of electrolytic differentiation in the rocks. Second, the outer part of the earth's crust is a very poor conductor so that it is doubtful whether even a high degree of electrification of the surface would have much effect on the interior. Third, electrolysis due to anysuch mild causes as we have here postulated must be an extremely slow process, too slow, presumably, to have any appreciable result within a month or two. Other objections join with these three in making it seem improbable that the sun's electrical activity has any direct effect upon movements of the earth's crust.The third, or meteorological hypothesis, which makes barometric pressure the main intermediary between solar activity and earthquakes, seems at first sight almost as improbable as the thermal and electrical hypotheses. Nevertheless, it has a certain degree of observational support of a kind which is wholly lacking in the other two cases. Among the extensive writings on the periodicity of earthquakes one main fact stands out with great distinctness: earthquakes vary in number according to the season. This fact has already been shown incidentally in the table of earthquake frequency by months. If allowance is made for the fact that February is a short month, there is a regular decrease in the frequency of severe earthquakes from December and January to June. Since most of Milne's earthquakes occurred in the northern hemisphere, this means that severe earthquakes occur in winter about 20 per cent oftener than in summer.TABLE 8SEASONAL MARCH OF EARTHQUAKESAFTER DAVISSON AND KNOTTABCDEFGRegionLimiting DatesNumber of ShocksMaximum MonthAmplitudeExpected AmplitudeRatio ofActual toExpected AmplitudeNorthern Hemisphere223-18505879Dec.0.1100.0234.8Northern Hemisphere1865-18848133Dec.0.2900.02014.5Europe1865-18845499Dec.0.3500.02414.6Europe306-18431961Dec.0.2200.0405.5Southeast Europe1859-18873470Dec.0.2100.0307.0Vesuvius District1865-1883513Dec.0.2500.0783.2Italy:Old Tromometre1872-188761732Dec.0.4900.00770.0Old Tromometre1876-188738546Dec.0.4600.00949.5Normal Tromometre1876-188738546Dec.0.4900.00952.8Balkan, etc.1865-1884624Dec.0.2700.0713.8Hungary, etc.1865-1884384Dec.0.3100.0903.4Italy1865-18832350Dec.(Sept.)0.1400.0373.8Grecian Archip.1859-18813578Dec.-Jan.0.1640.0305.5Austria1865-1884461Jan.0.3700.0834.4Switzerland, etc.1865-1883524Jan.0.5600.0777.3Asia1865-1884458Feb.0.3300.0834.0North America1865-1884552Nov.0.3500.0754.7California1850-1886949Oct.0.3000.0585.2Japan1878-1881246Dec.0.4600.1134.1Japan1872-1880367Dec.-Jan.0.2560.0932.8Japan1876-18911104Feb.0.1900.0533.6Japan1885-18892997Oct.0.0800.0322.5Zante1825-18631326Aug.0.1000.0492.0Italy, North of Naples1865-18831513Sept.(Nov.)0.2100.0464.6East Indies1873-1881515Aug., Oct., or Dec.?0.071?0.0780.9Malay Archip.1865-1884598May0.1900.0722.6New Zealand1869-1879585Aug.-Sept.0.2030.0732.8Chile1873-1881212July0.4800.1223.9Southern Hemisphere1865-1884751July0.3700.0655.7New Zealand1868-1890641March, May0.0500.0700.7Chile1865-1883?316July, Dec.0.2700.1002.7Peru, Bolivia1865-1884350July0.4800.0955.1The most thorough investigation of this subject seems to have been that of Davisson.[132]His results have been worked over and amplified by Knott,[133]who has tested them by Schuster's exact mathematical methods. His results are given in Table 8.[134]Here the northern hemisphereis placed first; then come the East Indies and the Malay Archipelago lying close to the equator; and finally the southern hemisphere. In the northern hemisphere practically all the maxima come in the winter, for the month of December appears in fifteen cases out of the twenty-five in column D, while January, February, or November appears in six others. It is also noticeable that in sixteen cases out of twenty-five the ratio of the actual to the expected amplitude in column G is four or more, so that a real relationship is indicated, while the ratio falls below three only in Japan and Zante. The equatorial data, unlike those of the northern hemisphere, are indefinite, for in the East Indies no month shows a marked maximum and the expected amplitude exceeds the actual amplitude. Even in the Malay Archipelago, which shows a maximum in May, the ratio of actual to expected amplitude is only 2.6. Turning to the southern hemisphere, the winter months of that hemisphere are as strongly marked by a maximum as are the winter months of the northernhemisphere. July or August appears in five out of six cases. Here the ratio between the actual and expected amplitudes is not so great as in the northern hemisphere. Nevertheless, it is practically four in Chile, and exceeds five in Peru and Bolivia, and in the data for the entire southern hemisphere.The whole relationship between earthquakes and the seasons in the northern and southern hemispheres is summed up in Fig. 12 taken from Knott. The northern hemisphere shows a regular diminution in earthquake frequency from December until June, and an increase the rest of the year. In the southern hemisphere the course of events is the same so far as summer and winter are concerned, for August with its maximum comes in winter, while February with its minimum comes in summer. In the southern hemisphere the winter month of greatest seismic activity has over 100 per cent more earthquakes than the summer month of least activity. In the northern hemisphere this difference is about 80 per cent, but this smaller figure occurs partly because the northern data include certain interesting and significant regions like Japan and China where the usual conditions are reversed.[135]If equatorial regions were included in Fig. 12, they would give an almost straight line.The connection between earthquakes and the seasons is so strong that almost no students of seismology question it, although they do not agree as to its cause. A meteorological hypothesis seems to be the only logical explanation.[136]Wherever sufficient data are available, earthquakesappear to be most numerous when climatic conditions cause the earth's surface to be most heavily loaded or to change its load most rapidly. The main factor in the loading is apparently atmospheric pressure. This acts in two ways. First, when the continents become cold in winter the pressure increases. On an average the air at sea level presses upon the earth's surface at the rate of 14.7 pounds per square inch, or over a ton per square foot, and only a little short of thirty million tons per square mile. An average difference of one inch between the atmospheric pressure of summer and winter over ten million square miles of the continent of Asia, for example, means that the continent's load in winter is about ten million million tons heavier than in summer. Second, the changes in atmospheric pressure due to the passage of storms are relatively sharp and sudden. Hence they are probably more effective than the variations in the load from season to season. This is suggested by the rapidity with which the terrestrial response seems to follow the supposed solar cause of earthquakes. It is also suggested by the fact that violent storms are frequently followed by violent earthquakes. "Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in the typhoon region of Japan, China, and the East Indies. During tropical hurricanes a change of pressure amounting to half an inch in two hours is common. On September22, 1885, at False Point Lighthouse on the Bay of Bengal, the barometer fell about an inch in six hours, then nearly an inch and a half in not much over two hours, and finally rose fully two inches inside of two hours. A drop of two inches in barometric pressure means that a load of about two million tons is removedfrom each square mile of land; the corresponding rise of pressure means the addition of a similar load. Such a storm, and to a less degree every other storm, strikes a blow upon the earth's surface, first by removing millions of tons of pressure and then by putting them on again.[137]Such storms, as we have seen, are much more frequent and severe when sunspots are numerous than at other times. Moreover, as Veeder[138]long ago showed, one of the most noteworthy evidences of a connection between sunspots and the weather is a sudden increase of pressure in certain widely separated high pressure areas. In most parts of the world winter is not only the season of highest pressure and of most frequent changes of Veeder's type, but also of severest storms. Hence a meteorological hypothesis would lead to the expectation that earthquakes would occur more frequently in winter than in summer. On the Chinese coast, however, and also on the oceanic side of Japan, as well as in some more tropical regions, the chief storms come in summer in the form of typhoons. These are the places where earthquakes also are most abundant in summer. Thus, wherever we turn, storms and the related barometric changes seem to be most frequent and severe at the very times when earthquakes are also most frequent.Fig. 12Fig. 12. Seasonal distribution of earthquakes. (After Davisson and Knott.)——  Northern Hemisphere.- - - - Southern Hemisphere.Other meteorological factors, such as rain, snow, winds, and currents, probably have some effect on earthquakesthrough their ability to load the earth's crust. The coming of vegetation may also help. These agencies, however, appear to be of small importance compared with the storms. In high latitudes and in regions of abundant storminess most of these factors generally combine with barometric pressure to produce frequent changes in the load of the earth's crust, especially in winter. In low latitudes, on the other hand, there are few severe storms, and relatively little contrast in pressure and vegetation from season to season; there is no snow; and the amount of ground water changes little. With this goes the twofold fact that there is no marked seasonal distribution of earthquakes, and that except in certain local volcanic areas, earthquakes appear to be rare. In proportion to the areas concerned, for example, there is little evidence of earthquakes in equatorial Africa and South America.The question of the reality of the connection between meteorological conditions and crustal movements is so important that every possible test should be applied. At the suggestion of Professor Schlesinger we have looked up a very ingenious line of inquiry. During the last decades of the nineteenth century, a long series of extremely accurate observations of latitude disclosed a fact which had previously been suspected but not demonstrated, namely, that the earth wabbles a little about its axis. The axis itself always points in the same direction, and since the earth slides irregularly around it the latitude of all parts of the earth keeps changing. Chandler has shown that the wabbling thus induced consists of two parts. The first is a movement in a circle with a radius of about fifteen feet which is described in approximately 430 days. This so-called Eulerian movement is a normal gyroscopic motion like the slow gyration of aspinning top. This depends on purely astronomical causes, and no terrestrial cause can stop it or eliminate it. The period appears to be constant, but there are certain puzzling irregularities. The usual amplitude of this movement, as Schlesinger[139]puts it, "is about 0".27, but twice in recent years it has jumped to 0".40. Such a change could be accounted for by supposing that the earth had received a severe blow or a series of milder blows tending in the same direction." These blows, which were originally suggested by Helmert are most interesting in view of our suggestion as to the blows struck by storms.The second movement of the pole has a period of a year, and is roughly an ellipse whose longest radius is fourteen feet and the shortest, four feet; or, to put it technically, there is an annual term with a maximum amplitude of about 0".20. This, however, varies irregularly. The result is that the pole seems to wander over the earth's surface in the spiral fashion illustrated in Fig. 13. It was early suggested that this peculiar wandering of the pole in an annual period must be due to meteorological causes. Jeffreys[140]has investigated the matter exhaustively. He assumes certain reasonable values for the weight of air added or subtracted from different parts of the earth's surface according to the seasons. He also considers the effect of precipitation, vegetation, and polar ice, and of variations of temperature and atmospheric pressure in their relation to movements of the ocean. Then he proceeds to compare allthese with the actual wandering of the pole from 1907 to 1913. While it is as yet too early to say that any special movement of the pole was due to the specific meteorological conditions of any particular year, Jeffreys' work makes it clear that meteorological causes, especially atmospheric pressure, are sufficient to cause the observed irregular wanderings. Slight wanderings may arise from various other sources such as movements of the rocks when geological faults occur or the rush of a great wave due to a submarine earthquake. So far as known, however, all these other agencies cause insignificant displacements compared with those arising from movements of the air. This fact coupled with the mathematical certainty that meteorological phenomena must produce some wandering of the pole, has caused most astronomers to accept Jeffreys' conclusion. If we follow their example we are led to conclude that changes in atmospheric pressure and in the other meteorological conditions strike blows which sometimes shift the earthseveral feet from its normal position in respect to the axis.Fig. 13Fig. 13. Wandering of the pole from 1890 to 1898.(After Moulton.)If the foregoing reasoning is correct, the great and especially the sudden departures from the smooth gyroscopic circle described by the pole in the Eulerian motion would be expected to occur at about the same time as unusual earthquake activity. This brings us to an interesting inquiry carried out by Milne[141]and amplified by Knott.[142]Taking Albrecht's representation of the irregular spiral-like motion of the pole, as given in Fig. 13, they show that there is a preponderance of severe earthquakes at times when the direction of motion of the earth in reference to its axis departs from the smooth Eulerian curve. A summary of their results is given in Table 9. The table indicates that during the period from 1892 to 1905 there were nine different times when the curve of Fig. 13 changed its direction or was deflected by less than 10° during a tenth of a year. In other words, during those periods it did not curve as much as it ought according to the Eulerian movement. At such times there were 179 world-shaking earthquakes, or an average of about 19.9 per tenth of a year. According to the other lines of Table 9, in thirty-two cases the deflection during a tenth of a year was between 10° and 25°, while in fifty-six cases it was from 25° to 40°. During these periods the curve remained close to the Eulerian path and the world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the deflection was high, that is, when meteorological conditions threw the earth far out of its Eulerian course, the earthquakes were again numerous, the number rising to 23.4 when the deflection amounted to more than 55°.TABLE 9DEFLECTION OF PATH OF POLE COMPARED WITH EARTHQUAKESDeflectionNo. of DeflectionsNo. of EarthquakesAverage No. of Earthquakes0-10°917919.910-25°322638.225-40°5672212.940-55°1936619.3over 55°716423.4In order to test this conclusion in another way we have followed a suggestion of Professor Schlesinger. Under his advice the Eulerian motion has been eliminated and a new series of earthquake records has been compared with the remaining motions of the poles which presumably arise largely from meteorological causes. For this purpose use has been made of the very full records of earthquakes published under the auspices of the International Seismological Commission for the years 1903 to 1908, the only years for which they are available. These include every known shock of every description which was either recorded by seismographs or by direct observation in any part of the world. Each shock is given the same weight, no matter what its violence or how closely it follows another. The angle of deflection has been measured as Milne measured it, but since the Eulerian motion is eliminated, our zero is approximately the normal condition which would prevail if there were no meteorological complications. Dividing the deflections into six equal groups according to the size of the angle, we get the result shown in Table 10.TABLE 10EARTHQUAKES IN 1903-1908 COMPARED WITH DEPARTURES OF THE PROJECTED CURVE OF THE EARTH'S AXIS FROM THE EULERIAN POSITIONAverage angle of deflection(10 periods of 1/10 year each)Average daily numberof earthquakes-10.5°8.3111.5°8.3525.8°8.2340.2°8.1454.7°8.8690.3°11.81Here where some twenty thousand earthquakes are employed the result agrees closely with that of Milne for a different series of years and for a much smaller number of earthquakes. So long as the path of the pole departs less than about 45° from the smooth gyroscopic Eulerian path, the number of earthquakes is almost constant, about eight and a quarter per day. When the angle becomes large, however, the number increases by nearly 50 per cent. Thus the work of Milne, Knott, and Jeffreys is confirmed by a new investigation. Apparently earthquakes and crustal movements are somehow related to sudden changes in the load imposed on the earth's crust by meteorological conditions.This conclusion is quite as surprising to the authors as to the reader—perhaps more so. At the beginning of this investigation we had no faith whatever in any importantrelation between climate and earthquakes. At its end we are inclined to believe that the relation is close and important.It must not be supposed, however, that meteorological conditions are thecauseof earthquakes and of movements of the earth's crust. Even though the load that the climatic agencies can impose upon the earth's crust runs into millions of tons per square mile, it is a trifle compared with what the crust is able to support. There is, however, a great difference between the cause and the occasion of a phenomenon. Suppose that a thick sheet of glass is placed under an increasing strain. If the strain is applied slowly enough, even so rigid a material as glass will ultimately bend rather than break. But suppose that while the tension is high the glass is tapped. A gentle tap may be followed by a tiny crack. A series of little taps may be the signal for small cracks to spread in every direction. A few slightly harder taps may cause the whole sheet to break suddenly into many pieces. Yet even the hardest tap may be the merest trifle compared with the strong force which is keeping the glass in a state of strain and which would ultimately bend it if given time.The earth as a whole appears to stand between steel and glass in rigidity. It is a matter of common observation that rocks stand high in this respect and in the consequent difficulty with which they can be bent without breaking. Because of the earth's contraction the crust endures a constant strain, which must gradually become enormous. This strain is increased by the fact that sediment is transferred from the lands to the borders of the sea and there forms areas of thick accumulation. From this has arisen the doctrine of isostasy, or of the equalization of crustal pressure. An important illustration ofthis is the oceanward and equatorial creep which has been described in Chapter XI. There we saw that when the lands have once been raised to high levels or when a shortening of the earth's axis by contraction has increased the oceanic bulge at the equator, or when the reverse has happened because of tidal retardation, the outer part of the earth appears to creep slowly back toward a position of perfect isostatic adjustment. If the sun had no influence upon the earth, either direct or indirect, isostasy and other terrestrial processes might flex the earth's crust so gradually that changes in the form and height of the lands would always take place slowly, even from the geological point of view. Thus erosion would usually be able to remove the rocks as rapidly as they were domed above the general level. If this happened, mountains would be rare or unknown, and hence climatic contrasts would be far less marked than is actually the case on our earth where crustal movements have repeatedly been rapid enough to produce mountains.Nature's methods rarely allow so gradual an adjustment to the forces of isostasy. While the crust is under a strain, not only because of contraction, but because of changes in its load through the transference of sediments and the slow increase or decrease in the bulge at the equator, the atmosphere more or less persistently carries on the tapping process. The violence of that process varies greatly, and the variations depend largely on the severity of the climatic contrasts. If the main outlines of the cyclonic hypothesis are reliable, one of the first effects of a disturbance of the sun's atmosphere is increased storminess upon the earth. This is accompanied by increased intensity in almost every meteorological process. The most important effect, however, so far as the earth's crust is concerned would apparently be the rapid andintense changes of atmospheric pressure which would arise from the swift passage of one severe storm after another. Each storm would be a little tap on the tensely strained crust. Any single tap might be of little consequence, even though it involved a change of a billion tons in the pressure on an area no larger than the state of Rhode Island. Yet a rapid and irregular succession of such taps might possibly cause the crust to crack, and finally to collapse in response to stresses arising from the shrinkage of the earth.Another and perhaps more important effect of variations in storminess and especially in the location of the stormy areas would be an acceleration of erosion in some places and a retardation elsewhere. A great increase in rainfall may almost denude the slopes of soil, while a diminution to the point where much of the vegetation dies off has a similar effect. If such changes should take place rapidly, great thicknesses of sediment might be concentrated in certain areas in a short time, thus disturbing the isostatic adjustment of the earth's crust. This might set up a state of strain which would ultimately have to be relieved, thus perhaps initiating profound crustal movements. Changes in the load of the earth's crust due to erosion and the deposition of sediment, no matter how rapid they may be from the geological standpoint, are slow compared with those due to changes in barometric pressure. A drop of an inch in barometric pressure is equivalent to the removal of about five inches of solid rock. Even under the most favorable circumstances, the removal of an average depth of five inches of rock or its equivalent in soil over millions of square miles would probably take several hundred years, while the removal of a similar load of air might occur in half a day or even a few hours. Thus the erosion and depositiondue to climatic variations presumably play their part in crustal deformation chiefly by producing crustal stresses, while the storms, as it were, strike sharp, sudden blows.Suppose now that a prolonged period of world-wide mild climate, such as is described in Chapter X, should permit an enormous accumulation of stresses due to contraction and tidal retardation. Suppose that then a sudden change of climate should produce a rapid shifting of the deep soil that had accumulated on the lands, with a corresponding localization and increase in strains. Suppose also that frequent and severe storms play their part, whether great or small, by producing an intensive tapping of the crust. In such a case the ultimate collapse would be correspondingly great, as would be evident in the succeeding geological epoch. The sea floor might sink lower, the continents might be elevated, and mountain ranges might be shoved up along lines of special weakness. This is the story of the geological period as known to historical geology. The force that causes such movements would be the pull of gravity upon the crust surrounding the earth's shrinking interior. Nevertheless climatic changes might occasionally set the date when the gravitative pull would finally overcome inertia, and thus usher in the crustal movements that close old geologic periods and inaugurate new ones. This, however, could occur only if the crust were under sufficient strain. As Lawson[143]says in his discussion of the "elastic rebound theory," the sudden shifts of the crust which seem to be the underlying cause of earthquakes "can occur only after the accumulation of strain to a limit and ... this accumulation involves a slow creep of the region affected.In the long periods between great earthquakes the energy necessary for such shocks is being stored up in the rocks as elastic compression."If a period of intense storminess should occur when the earth as a whole was in such a state of strain, the sudden release of the strains might lead to terrestrial changes which would alter the climate still further, making it more extreme, and perhaps permitting the storminess due to the solar disturbances to bring about glaciation. At the same time if volcanic activity should increase it would add its quota to the tendency toward glaciation. Nevertheless, it might easily happen that a very considerable amount of crustal movement would take place without causing a continental ice sheet or even a marked alpine ice sheet. Or again, if the strains in the earth's crust had already been largely released through other agencies before the stormy period began, the climate might become severe enough to cause glaciation in high latitudes without leading to any very marked movements of the earth's crust, as apparently happened in the Mid-Silurian period.CONCLUSIONHere we must bring this study of the earth's evolution to a close. Its fundamental principle has been that the present, if rightly understood, affords a full key to the past. With this as a guide we have touched on many hypotheses, some essential and some unessential to the general line of thought. The first main hypothesis is that the earth's present climatic variations are correlated with changes in the solar atmosphere. This is the keynote of the whole book. It is so well established, however,that it ranks as a theory rather than as an hypothesis. Next comes the hypothesis that variations in the solar atmosphere influence the earth's climate chiefly by causing variations not only in temperature but also in atmospheric pressure and thus in storminess, wind, and rainfall. This, too, is one of the essential foundations on which the rest of the book is built, but though this cyclonic hypothesis is still a matter of discussion, it seems to be based on strong evidence. These two hypotheses might lead us astray were they not balanced by another. This other is that many climatic conditions are due to purely terrestrial causes, such as the form and altitude of the lands, the degree to which the continents are united, the movement of ocean currents, the activity of volcanoes, and the composition of the atmosphere and the ocean. Only by combining the solar and the terrestrial can the truth be perceived. Finally, the last main hypothesis of this book holds that if the climatic conditions which now prevail at times of solar activity were magnified sufficiently and if they occurred in conjunction with certain important terrestrial conditions of which there is good evidence, they would produce most of the notable phenomena of glacial periods. For example, they would explain such puzzling conditions as the localization and periodicity of glaciation, the formation of loess, and the occurrence of glaciation in low latitudes during Permian and Proterozoic times. The converse of this is that if the conditions which now prevail at times when the sun is relatively inactive should be intensified, that is, if the sun's atmosphere should become calmer than now, and if the proper terrestrial conditions of topographic form and atmospheric composition should prevail, there would arise the mild climatic conditions which appear to have prevailed during the greater part of geologicaltime. In short, there seems thus far to be no phase of the climate of the past which is not in harmony with an hypothesis which combines into a single unit the three main hypotheses of this book, solar, cyclonic, and terrestrial.Outside the main line of thought lie several other hypotheses. Several of these, as well as some of the main hypotheses, are discussed chiefly inEarth and Sun, but as they are given a practical application in this book they deserve a place in this final summary. Each of these secondary hypotheses is in its way important. Yet any or all may prove untrue without altering our main conclusions. This point cannot be too strongly emphasized, for there is always danger that differences of opinion as to minor hypotheses and even as to details may divert attention from the main point. Among the non-essential hypotheses is the idea that the sun's atmosphere influences that of the earth electrically as well as thermally. This idea is still so new that it has only just entered the stage of active discussion, and naturally the weight of opinion is against it. Although not necessary to the main purpose of this book, it plays a minor rôle in the chapter dealing with the relation of the sun to other astronomical bodies. It also has a vital bearing on the further advance of the science of meteorology and the art of weather forecasting. Another secondary hypothesis holds that sunspots are set in motion by the planets. Whether the effect is gravitational or more probably electrical, or perhaps of some other sort, does not concern us at present, although the weight of evidence seems to point toward electronic emissions. This question, like that of the relative parts played by heat and electricity in terrestrial climatic changes, can be set aside for the moment. What does concern us is a third hypothesis, namely, thatif the planets really determine the periodicity of sunspots, even though not supplying the energy, the sun in its flight through space must have been repeatedly and more strongly influenced in the same way by many other heavenly bodies. In that case, climatic changes like those of the present, but sometimes greatly magnified, have presumably arisen because of the constantly changing position of the solar system in respect to other parts of the universe. Finally, the fourth of our secondary hypotheses postulates that at present the date of movements of the earth's crust is often determined by the fact that storms and other meteorological conditions keep changing the load upon first one part of the earth's surface and then upon another. Thus stresses that have accumulated in the earth's isostatic shell during the preceding months are released. In somewhat the same way epochs of extreme storminess and rapid erosion in the past may possibly have set the date for great movements of the earth's crust. This hypothesis, like the other three in our secondary or non-essential group, is still so new that only the first steps have been taken in testing it. Yet it seems to deserve careful study.In testing all the hypotheses here discussed, primary and secondary alike, the first necessity is a far greater amount of quantitative work. In this book there has been a constant attempt to subject every hypothesis to the test of statistical facts of observation. Nevertheless, we have been breaking so much new ground that in many cases exact facts are not yet available, while in others they can be properly investigated only by specialists in physics, astronomy, or mathematics. In most cases the next great step is to ascertain whether the forces here called upon are actually great enough to produce the observed results. Even though they act only as a meansof releasing the far greater forces due to the contraction of the earth and the sun, they need to be rigidly tested as to their ability to play even this minor rôle. Still another line of study that cries aloud for research is a fuller comparison between earthquakes on the one hand and meteorological conditions and the wandering of the poles on the other. Finally, an extremely interesting and hopeful quest is the determination of the positions and movements of additional stars and other celestial bodies, the faint and invisible as well as the bright, in order to ascertain the probable magnitude of their influence upon the sun and thus upon the earth at various times in the past and in the future. Perhaps we are even now approaching some star that will some day give rise to a period of climatic stress like that of the fourteenth century, or possibly to a glacial epoch. Or perhaps the variations in others of the nearer stars as well as Alpha Centauri may show a close relation to changes in the sun.Throughout this volume we have endeavored to discover new truth concerning the physical environment that has molded the evolution of all life. We have seen how delicate is the balance among the forces of nature, even though they be of the most stupendous magnitude. We have seen that a disturbance of this balance in one of the heavenly bodies may lead to profound changes in another far away. Yet during the billion years, more or less, of which we have knowledge, there appears never to have been a complete cataclysm involving the destruction of all life. One star after another, if our hypothesis is correct, has approached the solar system closely enough to set the atmosphere of the sun in such commotion that great changes of climate have occurred upon the earth. Yet never has the solar system passed so close to any other body or changed in any other way sufficientlyto blot out all living things. The effect of climatic changes has always been to alter the environment and therefore to destroy part of the life of a given time, but with this there has invariably gone a stimulus to other organic types. New adaptations have occurred, new lines of evolutionary progress have been initiated, and the net result has been greater organic diversity and richness. Temporarily a great change of climate may seem to retard evolution, but only for a moment as the geologist counts time. Then it becomes evident that the march of progress has actually been more rapid than usual. Thus the main periods of climatic stress are the most conspicuous milestones upon the upward path toward more varied adaptation. The end of each such period of stress has found the life of the world nearer to the high mentality which reaches out to the utmost limits of space, of time, and of thought in the search for some explanation of the meaning of the universe. Each approach of the sun to other bodies, if such be the cause of the major climatic changes, has brought the organic world one step nearer to the solution of the greatest of all problems,—the problem of whether there is a psychic goal beyond the mental goal toward which we are moving with ever accelerating speed. Throughout the vast eons of geological time the adjustment of force to force, of one body of matter to another, and of the physical environment to the organic response has been so delicate, and has tended so steadily toward the one main line of mental progress that there seems to be a purpose in it all. If the cosmic uniformity of climate continues to prevail and if the uniformity is varied by changes as stimulating as those of the past, the imagination can scarcely picture the wonders of the future. In the course of millions or even billions of years the development of mind, and perhaps of soul, many excelthat of today as far as the highest known type of mentality excels the primitive plasma from which all life appears to have arisen.INDEX* Indicates illustrations.Abbot, C. G., cited,45,52,237,238,239.Aboskun,104.Africa, earthquakes,301;East,seeEast Africa;lakes,143;North,seeNorth Africa.African glaciation,266.Air,seeAtmosphere.Alaska, glacial till in,287;Ice Age in,221.Albrecht, cited,304.Alexander, march of,88f.Allard, H. A., cited,183,184.Alpha Centauri, companion of,280;distance from sun,262;luminosity,278;speed of,281;variations,282.Alps, loess in,159;precipitation in,141;snow level in,139.Altair, companion of,280;luminosity,278;speed of,281.Amazon forest, temperature,17.Ancylus lake,217.Andes, snow line,139.Animals, climate and,1.Antarctica, mild climate,219;thickness of ice in,125;winds,135,161.Anti-cyclonic hypothesis,135ff.Appalachians, effect on ice sheet,121.Arabia, civilization in,67.Aral, Sea of,108.Archean rocks,211.Archeozoic,3f.;climate of,267.Arctic Ocean, submergence,219.Arctowski, H., cited,29,46,244.Argon, increase of,236.Arizona, rainfall,89,108;trees measured in,73.Arrhenius, S., cited,36,254.Arsis, of pulsation,24.Asbjörn Selsbane, corn of,101.Asia, atmospheric pressure,298;central,changes of climate, *75;central, post-glacial climate,271;climate,66;glaciation in,131;storminess in,60;western, climate in,84f.Atlantic Ocean, storminess,57.Atmosphere, changes,19f.,229;composition of,223-241;effect on temperature,231.Atmospheric circulation, glaciation and,42.Atmospheric electricity, solar relations of,56.Atmospheric pressure, earthquakes and,298;evaporation and,237;increase in,239;redistribution of,49;variation,53.Australia, East, mild climate,219;precipitation,144.Axis, earth's,48;wabbling of,301.Bacon, Sir Francis, cited,27.Bacubirito, meteor at,246.Baltic Sea, as lake,217;freezing of,100;ice,26;storm-floods,99;submergence,219.Bardsson, Ivar,106.Barkow, cited,135.Barometric pressure, solar relations of,56.Barrell, J., cited,3,200,213,234.Bartoli, A. G., cited,257.Bauer, L. A., cited,150.Beaches, under water,97.Beadnell, H. J. L., cited,143.Beluchistan, rainfall,89.Bengal, Bay of, cyclones in,149.Bengal, famine in,104f.Berlin, rainfall and temperature,93.Betelgeuse,259f.;distance from sun,262.Bible, climatic evidence in,91f.;palms in,92.Binary stars,252.Birkeland, K., cited,244.Black Earth region, loess in,159.Boca, Cal., correlation coefficients,83,85.Boltzmann, L., cited,257.Bonneville, Lake,142,143.Borkum, storm-flood in,99.Boss, L. cited,268,269.Botanical evidence of mild climates,167ff.Boulders, on Irish coast,119.Bowie, W., cited,293.Bowman, I., cited,213.Britain, forests,220;level of land,220.British Isles, height of land,111;temperature,216.Brooks, C. E. P., cited,115,143,196,215,225.Brooks, C. F., cited,209.Brown, E. W., cited,191,244.Brückner, E., cited,27.Brückner periods,27f.Bufo, habitat of,202.Buhl stage,216.Bull, Dr., cited,100,101.Butler, H. C., cited,66,67ff.,70,76.California, changes of climate, *75;correlations of rainfall,86;measurements of sequoias in,73,74ff.;rainfall,108.Cambrian period,4f.Canada, storminess,53f.,57;storm tracks in,113.Cape Farewell, shore ice at,105.Carbon dioxide, erosion and,119f.;from volcanoes,23;hypothesis,139;importance of,9,11f.;in Permian,148;.in atmosphere,20,96,238;in ocean,226;nebular hypothesis and,232;theory of glaciation,36ff.Caribbean mountains, origin of,193.Carnegie Institution of Washington,74.Caspian Sea, climatic stress,104;rainfall,107f.;rise and fall,27;ruins in,71.Cenozoic, climate,266;fossils,21.Central America, Maya ruins,95.Chad, Lake, swamps of,171.Chamberlin, R. T., cited,166,233,269.Chamberlin, T. C., cited,19,36,38,39,42f.,48,122,125,152,156,190,195,227,269.Chandler, S. C., cited,301.Chinese earthquakes, periodicity of,245.Chinese, sunspot observations,108f.Chinese Turkestan, desiccation in,66.Chronology, glacial,215.Clarke, F. W., cited,226,235.Clayton, H. H., cited,173f.Climate, effect of contraction,189ff.;affect of salinity,224;in history,64-97;uniformity,1-15;variability,16-32.Climates, mild, causes of,166-187;mild, periods of,274.Climatic changes, and crustal movements,285ff.;hypotheses of,33-50;mountain-building and, *25;post-glacial crustal movements and,215-222;terrestrial causes of,188-214.Climatic sequence,16f.Climatic stages, post-glacial,270.Climatic stress, in fourteenth century,98-109.Climatic uniformity, hypothesis of,65,71f.Climatic zoning,169.Cloudiness, glaciation and,114,147.Clouds, as protection,197.Colfax, Cal., correlation coefficients,83.Cologne, flood at,99.Compass, variations,150.Continental climate, variations,103.Continents, effect on climate,111f.Contraction, effect on climate,189ff.,199,207;effect on lands,207;heat of sun and,13f.;irregular,195;of the earth,18;of the sun,249;stresses caused by,310.Convection, carbon dioxide and,239.Corals, in high latitudes,21,39,167,178.Cordeiro, F. J. B., cited,181,183,186.Correlation coefficients, earthquakes and sunspots,291;Jerusalem rainfall and sequoia growth,83ff.;rainfall and tree growth,79ff.Cosmos, effect of light,185.Cressey, G. B., cited,80.Cretaceous, lava,211;mountain ranges,44;paleogeography, *201;submergence of North America,200.Croll, J., cited,34ff.,176.Croll's hypothesis, snow line,139.Crust, climate and movements of,63,287,310;movements of,43;strains in,22.Currents and planetary winds,174.Cycads,169.Cyclonic hypothesis,97;loess and,163;Permian glaciation and,148;snow line,139.Cyclonic storms, in glacial epochs,140f.;solar electricity and,243(seeStorms, Storminess).Cyclonic vacillations,30f.;nature of,57ff.Daily vibrations,28f.Danube, frozen,98.Darwin, G. H., cited,191.Daun stage,217.Davis, W. M., cited,271.Davisson, C., cited,294,295,299.Day, C. P., cited,239.Day, length of,18,191.Dead Sea, palms near,92.Death Valley,142.De Ballore, M., cited,297,298.Deep-sea circulation, rapidity,227;salinity and,176;solar activity and,179.De Geer, S., cited,215,221.De Lapparent, A., cited,200.Denmark, fossils,271."Desert pavements,"161.Deserts, abundant flora of,171;and pulsations theory,88ff.;red beds of,170.Devonian, climate,266;mountains,209.Dog, climate and,1.Donegal County, Ireland,220.Double stars,272,280;electrical effect of,261.Douglass, A. E., cited,28,73,74f.,84,85,107.Dragon Town, destruction of,104,7108.Drake, N. F., cited,297,298.Droughts, and pulsations theory,87f.;in England,102;in India,104f.Drumkelin Bog, Ireland, log cabin in,220.Dust, at high levels,240.Earth, crust of and the sun,285-317;internal heat,212;nature of mild climate,274;position of axis,181;rigidity of,307;temperature gradient,213;temperature of surface,8.Earthquakes, and seasons,294,297;and sunspots,288f.;and tropical hurricanes,300;and wandering of pole,304f.;cause of,307;compared with departures from Eulerian position,306;seasonal distribution of,299;seasonal march,295."Earthquake weather,"298.East Africa, mild climate,219.East Indies, earthquakes of,296.Eberswalde, tree growth at,102f.Ecliptic, obliquity of,217.Electrical currents, in solar atmosphere,261.Electrical emissions, variation of,275.Electrical hypothesis,150,250f.,256ff.Electrical phenomena, storminess and,56.Electricity, and earthquakes,292;solar,243.Electro-magnetic hypothesis,244.Electrons, solar,56;variation of,256.Electro-stellar hypothesis,274.Elevation, climatic changes and,39.Engedi, palms in,92.England, climatic stress,101f.;storminess and rainfall,107.Eocene, climate,266.Equinoxes, precession of,96.Erosion, storminess and,309.Eskimo, in Greenland,106.Eulerian movement,301,304.Euphrates,67.Europe, climatic stress,98ff.,102f.;climatic table,215;glaciation in,131,ice sheet,121;inundations of rivers,99;post-glacial climate,271;rainfall,107;submergence,196,200.Evaporation, and glaciation,112,114;atmospheric pressure and,237;from plants,179;importance,129;in trade-wind belt,117;rapidity of,224.Evening primrose, effect of light,184.Evolution, climate and,20;geographical complexity and,241;glaciation and,33;of the earth,311.Faculæ, cause of,61.False Point Lighthouse, barometric pressure at,299.Famine, cause of,103;in England,101f.;in India,104f.;pulsations theory and,87f.Faunas, and mild climates,168f.;in Permian,152f.Fennoscandian pause,216.Flowering, light and,184.Fog, and glaciation,116;as protection,197;temperature and,178.Forests, climate and,66.Form of the land,43ff.Fossil floras, and mild climates,168;in Antarctica,273;in Greenland,273.Fossils,169,230;and loess,158;Archeozoic,3f.;Cenozoic,21;dating of,153;glaciation and,138;in peat bogs,271;mild climate,167;Proterozoic,4,6f.Fourteenth century, climatic stress in,98-109.Fowle, F. E., cited,45,237,238,239.Frech, F., cited,36.Free, E. E., cited,142.Freezing, salinity and,224.Fresno, rainfall record,82."Friction variables,"247.Frisian Islands, storm-flood,99.Fritz, H., cited,109.Frogs, distribution of,202.Fuchs, cited,289.Galaxy,252.Galveston, Tex., rainfall and temperature,94.Garner, W. W., cited,183,184.Gasses, in air,233.Geographers, and climatic changes,65ff.Geological time table, *5.Geologic oscillations,18f.,21ff.,188,240.Geologists, changes in ideas of,64f.Germanic myths,219.Germany, forests,220;growth of trees in,102;storms in,102.Gilbert, G. K., cited,143.Glacial epochs, causes of,268;dates of,216;intervals between,264f.;length of,166f.Glacial fluctuations,24ff.;nature of,57ff.Glacial period, at present,272;ice in,57f.;length of,269;list,265;temperature,38.Glaciation, and loess,155f.;and movement of crust,287;conditions favorable for,111,extent of,124;hypotheses of,33ff.;in southern Canada,18;localization of,130ff.;Permian, *145;solar-cyclonic hypothesis of,110-129;suddenness of,138;upper limit of,141.Goldthwait, J. W., cited,271.Gondwana land,21,204.Gravitation, effect on sun,250;pull of,244.Great Basin, in glacial period,126;salt lakes in,142.Great Ice Age, see Pleistocene.Great Plains, effect on ice sheet,120.Greenland, climatic stress,105ff.;ice,26;rainfall,7108;storminess,57;submergence,219;vegetation,21,37,287;winds,135,161.Gregory, J. W., cited,90ff.,97.Gschnitz stage,216.Guatemala, ruins in,95.Guervain, cited,135.Gyroscope, earth as,181.Hale, G. E., cited,56,62.Hamdulla, cited,104.Hann, J., cited,66.Hansa Union, operations of,100.Harmer, F. W., cited,115,119.Heat, and earthquakes,292;earth's internal,18.Hedin, S., cited,88.Heim, A., cited,190.Heligoland, flood in,99.Helland-Hansen, B., cited,174.Helmert, F. R., cited,302.Henderson, L. J., cited,9,10,11,12.Henry, A. J., cited,94,208.Hercynian Mountains,45.High pressure and glaciation,115,135.Himalayas, glaciation,144;origin of,193,snow line,139.Himley, cited,104.Historic pulsations,24f.;nature of,57ff.History, climate of,64-97;climatic pulsations and,26.Hobbs, W. H., cited,115,125,135,161.Hot springs, temperature of,6.Humphreys, W. J., cited,2,37f.,45,46,50,56,238.Hurricanes, in arid regions,144;sunspots and,53.Hyades, cluster in,268.Ice, accumulations,57f.;advances of,122;distribution of,131,drift,105.Ice sheets, disappearance,128;limits,120;localization,130ff.;rate of retreat,165;thickness,125.Iceland, submergence,219.Iowan ice sheet, rapid retreat,165.Iowan loess,158.India, drought,104f.;famine,104f.;rainfall,7108.Indian glaciation,266.Inter-glacial epoch, Permian,153.Internal heat of earth,212.Ireland, Drumkelin Bog,220;in glacial period,119;level of land,220;storminess and rainfall,107;submergence,219.Irish Sea, tides,191.Irrigation ditches, abandoned,97.Isostasy,307ff.Italy, southern, climate of,86f.Japan, earthquakes of,296.Javanese mountains, origin of,193.Jaxartes,7108.Jeans, J. H., cited,251,252,253,266,272.Jeffreys, H., cited,302,303,306.Jeffreys, J., cited,191.Jericho, palms in,92.Jerusalem, rainfall,86;rainfall and temperature,94;rainfall in, and sequoia growth,83ff.Johnson, cited,226.Judea, palms in,92.Jupiter, and sunspots,243;effect of,253;periodicity of,61f.;temperature of,258;tidal effect of,250.Jurassic, climate,266;mountain ranges,44.Kansas, variations of seasons,103.Kara Koshun marsh, Lop Nor,104.Keewatin center,113;evaporation in,129.Keewatin ice sheet,121.Kelvin, Lord, cited,13f.Keyes, C. R., cited,156.Kirk, E., cited,287.Knott, C. G., cited,294,295,297,299,304,306.Knowlton, F. H., cited167,169,170,212,232.Köppen, W.,,52,140.Krakatoa, glaciation and,48;volcanic hypothesis and,45.Krümmel, O., cited,224,228.Kullmer, C. J., cited,113,115,128;map of storminess, *54.Kungaspegel, sea routes described,106.Labor, price in England,102.Labradorean center of glaciation,113.Lahontan, Lake,142.Lake strands,seeStrands.Lake Superior, lava,211.Lakes, during glacial periods,141f.;in semi-arid regions,60;of Great Basin,126;ruins in,97.Land, and water, climatic effect of,196ff.;distribution of,200, form of,43ff.;range of temperature and,196.Lavas, climatic effect of,211.Lawson, A. C., cited,310.Lebanon, cedars of,83.Leiter, H., cited,71.Leverett, F., cited,271.Life, atmosphere and,229f.;chemical characteristic of,12;effect of salinity,225;of glacial period,127;persistence of forms,230.Light, effect of atmosphere on,236;effect on plants,184ff.;ultra-violet, storminess and,56;variation of,275.Litorina sea,218.Loess, date of,156ff.;origin of,155,165.Lop Nor, rise of,104;swamps,171.Lows, and glacial lobes,122;movements of,126;see Storms and Cyclones.Lulan,104.Lull, R. S., cited,5,188.MacDougal, D. T., cited,171.McGee, W. J., cited,156.Macmillan, W. D., cited,191.Magdalenian period,216.Magnetic fields of sunspots,56.Magnetic poles, relation to storm tracks,150.Makran, climate,89;rainfall,89.Malay Archipelago, earthquakes of,296.Mallet, R., cited,288.Malta, rainfall,86.Manson, M., cited,147.Mayas, civilization,26;ruins,95.Mayence, flood at,99.Mazelle, E., cited,224.Mediterranean, climate of,72;rainfall records,86;storminess in,60.Mercury, and sunspots,243.Mesozoic, climate,266;crustal changes,286;emergence of lands,287.Messier,8;variables,248.Metcalf, M. M., cited,202.Meteorological factors and earthquakes,300f.Meteorological hypothesis of crustal movements,294.Meteors, and sun's heat,13,246.Michelson, A. A., cited,259.Middle Silurian, fauna in Alaska,287.Mild climates,seeClimates, mild.Milky Way,252.Mill, H. R., cited,228.Milne, J., cited,288,290,294,304,306.Miocene, crustal changes,287.Mississippi Basin, loess in,159.Mogul emperor, and famine,104.Monsoons, character of,146;direction of,208;Indian famines and,105.Moulton, F. R., cited,13,258,269.Mountain building, climatic changes and, *25.Mountains, folding of,190;rainfall, on,208.Multiple stars,252.Nansen, F., cited,122,174.Naples, rainfall,86.Nathorst, cited,169.Nebulæ,247.Nebular hypothesis,232,267.Neolithic period,218.Nevada, correlations of rainfall,86.New England, height of land,111.New Mexico, rainfall,89.New Orleans, La., rainfall and temperature,94.New Zealand, climate,177;tree ferns,179.Newcomb, S., cited,52.Nile floods, periodicity in,245.Nitrogen, in atmosphere,19.Niya, Chinese Turkestan, desiccation at,66.Nocturnal cooling, changes in,238f.Norlind, A., cited,100.Norsemen, route to Greenland,26.Norse sagas,219.North Africa, climate of,71;Roman aqueducts in,71.North America, at maximum glaciation,122ff.;emergence of lands,193,glaciation in,131,height of land,111,interior sea in,200,inundations,196,loess in,155,submergence of lands,19,21.North Atlantic Ocean, salinity,228.North Sea, climatic stress,98ff.;floods around,26,99;rainfall,107;storminess,57.Northern hemisphere, earthquakes of,294.Norway, decay,100;temperature,177.Novæ,247.Oceanic circulation, carbon dioxide and,39ff.Oceanic climate, characteristics,103.Oceanic currents, diversion,44;influence of land distribution,203.Oceans, age of,223;composition of,223-241;deepening of,199;salinity,19,223;temperature,6,152,180,226.Okada, T., cited,224.Old Testament, temperature,92.Orbital precessions,27.Ordovician, climate,266.Organic evolution, glacial fluctuations and,26.Orion, nebulosity near,247;stars near,248.Orontes,67.Osborn, H. F., cited,216.Owens-Searles, lakes,142.Oxus,7108.Oxygen, in atmosphere,20,234;in Permian,152.Ozone, cause of,56.Paleolithic,216.Paleozoic, climate,266;mountains in,209.Palestine, change of climate,91f.Palms, climatic change and,91f.;in Ireland,179.Palmyra, ruins of,66.Parallaxes of stars,276f.Patrician center,134.Peat-bog period, first,218.Penck, A., cited,139,156,157,158,269.Pennsylvanian, life of,26.Periodicities,245f.Periodicity, of climatic phenomena,60f.;of glaciation,268;of sunspots,243.Permian, climate,266;distribution of glaciation,152;glaciation,60,144, *145,226;glaciation and mountains,45;life of,26;red beds,151;temperature,146f.Perry, cited,289.Persia, lakes,143;rainfall,89.Pettersson, O., cited,98ff.,100f.,103,106,219.Pirsson, L. V., cited,3,196.Planetary hypothesis,253,267.Planetary nebulæ,252.Planets, and sunspots,243;effect of star on,255;sunspot cycle and,62;temperatures,8f.Plants, climate and,1f.;effect of light,184ff.Pleion, defined,29.Pleionian migrations,29f.Pleistocene, climate,266;duration of,48;glaciation,110ff.;ice sheets, *123.Pluvial climate, causes of,143;during glacial periods,141.Po, frozen,98.Polaris,272.Polar wandering, hypothesis of,48f.Pole and earthquakes,305.Post-glacial crustal movements and climatic changes,215-222.Poynting, J. H., cited,8.Precessional hypothesis,34f.Precipitation, and glaciation,114,133;during glacial period,118;snow line and,139;temperature and,94.Procyon, companion of,280;luminosity,278;speed of,281.Progressive change,241.Progressive desiccation, hypothesis of,65ff.Proterozoic,4f.;fossils,6f.;glaciation,18,144,226,266;lava,211;mountains in,209;oceanic salinity,42f.;oxygen in air,234;red beds,151;temperature,146f.Pulsations, hypothesis of,65,72ff.Pulsatory climatic changes,72ff.Pulsatory hypothesis,272.Pumpelly, R., cited,271.Radiation, variation of,275.Radioactivity, heat of sun and,14f.Rainfall, changes in,93f.;glaciation and,50;sunspots and,53, *58,59;tree growth and,79.Red beds,151,170.Rhine, flood,99;frozen,98.Rho Ophiuchi, variables,248."Rice grains,"61.Richardson, O. W., cited,256.Rigidity, of earth,307.Roads, climate and,66.Rogers, Thorwald, cited,101.Romans, aqueduct of,71.Rome, history of,87.Rotation, of earth,18f.Ruden, storm-flood,99.Rugen, storm-flood,99.Ruins, as climatic evidence,66;rainfall and,60.Sacramento, correlation coefficients,82f.,85;rainfall,86;rainfall record,79.Sagas, cited,105f.St. John, C. E., cited,236.Salinity, deep-sea circulation and,176;effect on climate,224;in North Atlantic,228;ocean temperature and,226;of ocean,19,120.Salisbury, R. D., cited,111,125,129,139,156,206,269,271.Salt, in ocean,223.San Bernardino, correlation of rainfall,85.Saturn, and sunspots,243;sunspot cycle and,62.Sayles, R. W., cited,183.Scandinavia, climatic stress,100f.;fossils,271;post-glacial climate,271;rainfall,107;storminess,57,107;temperature,216.Scandinavian center of glaciation,113.Schlesinger, F., cited,275,278,298,301,305.Schuchert, C., cited,3,5,23, *25, *123,138, *145,168,169,172,188,193,196,198,200, *201,206,211,230,265.Schuster, A., cited,61,244,294,296.Sculpture, Maya,96.Sea level and glaciation,119.Seasonal alternations,28f.Seasonal banding,183f.Seasonal changes, geological,183.Seasons, and earthquakes,294,295,297,299;evidences of,169.Secular progression,17ff.,188.Seistan, swamps,171.Sequoias, measurements of,74ff.;rainfall record,79.Setchell, W. A., cited,1.Shackleton, E., cited,125.Shapley, H., cited,246,247,254,256,275.Shimek, E., cited,157,161.Shreveport, La., rainfall and temperature,93f.Shrinkage of the earth,190.Siberia, and glaciation,132.Sierras, rainfall records,82.Simpson, G. C., cited,222.Sirius, companion of,280;distance from sun,262;luminosity,278;speed of,281.Slichter, C. S., cited,192.Smith, J. W., cited,73.Snowfall, glaciation and,50,114.Snowfield, climatic effects of,115.Snow line, height of,138;in Andes,139;in Himalayas,139.Solar activity, cycles of,245;deep-sea circulation and,179;ice and,134.Solar constant,114.Solar-cyclonic hypothesis,51-63,287;glaciation and,110-129.Solar prominences, cause of,61.Solar system,252;conservation of,243;proximity to stars,63.Solar variations, storms and,31.South America, earthquakes,301.South Pole, thickness of ice at,125.Southern hemisphere, earthquakes,296;glaciation in,131f.Southern Pacific railroad, rainfall records along,82.Soy beans, effect of light,185f.Space, sun's journey through,264-284.Spiral nebulæ,251f.;universe of,267.Spitzbergen, submergence,219.Springs, climate and,66.Stars, approach to sun,253;binary,252;clusters,252,268;effect on solar atmosphere,63;dark,254;parallaxes of,276f.;tidal action of,249.Stefan's Law,257.Stein, M. A., cited,78.Stellar approaches, probability of,260.Storm belt in arid regions,144.Storm-floods, in fourteenth century,99.Storminess, and erosion,309;and ice,134;effect on glaciation,112;sunspots and,163;temperature and,94,173.Storms, blows of,300,302;increase,60;movement of,125f.;movement of water and, *175;origin of,30f.;sunspots and,28,53;seeCyclones and Lows.Storm tracks, during glacial period,117;location,113;relation to magnetic poles,150;shifting of,119.Strands, climate and,66;in semi-arid regions,60;of salt lakes,142.Suess, E., cited,192.Sun, and the earth's crust,285-317;approach to star,253;atmosphere of,61,274;atmosphere of, and weather,52;cooling of,49;contraction of,249;disturbances of,172;effect of other bodies on,242-263;heat,13;journey through space,264-284;Knowlton's hypothesis of,168.Suncracks,232.Sunspot cycles,27f.Sunspots, and earthquakes,289;causes of,61;magnetic field of,261;maximum of,109;mild climates and,172;number,7108f.;periodicity,243;planetary hypothesis of,253;records,245;storminess and,163;storms and,300;temperature of earth and,52,173.Sunspot variations,282.Swamps, as desert phenomena,171.Sylt, storm-flood,99.Syria, civilization in,67;inscriptions in,76;Roman aqueducts in,71.Syrian Desert, ruins in,66.Talbert, cited,213.Tarim Basin, red beds,151.Tarim Desert, desiccation,66.Tarim River, swamps,171.Taylor, G., cited,140,144,191,271.Temperature, change of in Atlantic,174;changes in,93;climatic change and,49;critical,9;geological time and,3;glacial period,38;glaciation and,42,132,139;gradient of earth,213;of ocean,180;in Norway,177;in Permian,146f.;in Proterozoic,146f.;limits,6ff.;precipitation and,94;range of,3,8;solar activity and,140;storminess and,94,112,173;sunspots and,28,173;volcanic eruptions and,46;zones,172.Terrestial causes of climatic changes,188-214.Tertiary, lava,211.Thames, frozen,98.Thermal solar hypothesis,49f.,97.Thermo-pleion, movements of,30.Thesis, of pulsations,24.Thiryu, storm-flood,99.Tian-Shan Mountains, irrigation in,71.Tidal action of stars,249.Tidal effect, of Jupiter,253;of planets,244.Tidal hypothesis,251.Tidal retardation, effect on land and sea,191;rotation of earth and,18f.;stress caused by,310.Tides, cycles of,219.Time, geological,seeGeological time.Toads, distribution of,202.Tobacco plant, effect of light,184.Topography, and glaciation,132.Transcaspian Basin, red beds,151.Tree ferns, in New Zealand,179.Tree growth, periodicity in,245;rainfall and,79.Tress, in California,219;measurement of,73ff.Triassic, climate,266.Trifid Nebula, variables,248.Trondheim, wheat in,101.Trondhenäs, corn in,101.Tropical cyclones, in glacial epochs,140f.;occurrence,148;solar activity and,113.Tropical hurricanes, earthquakes and,300;sunspots and,149.Turfan, temperature,17.Turner, H. H., cited,245.Tyler, J. M., cited,216.Tyndall, J., cited,36,37.Typhoon region, "earthquake weather,"298.Typhoons, occurrence,300.United States, rainfall and temperature in Gulf region,93f.;salt lakes in,142;southwestern, climate,66;storminess,53f.,60.Variables,247.Veeder, M. A., cited,300.Vegetation, theory of pulsations and,90.Venus, atmosphere of,236.Vesterbygd, invasion of,106.Vicksburg, Miss., rainfall and temperature,93f.Volcanic activity, climate and,210;movement of the earth's crust and,285;times of uplifting lands and,23.Volcanic dust, climatic changes and,97.Volcanic hypothesis, climatic change and,45ff.;snow line,139.Volcanoes, activity of,96.Volga,108.Walcott, C. D., cited,4,230.Wandering of the pole,302.Water, importance,9.Water vapor, condensation of,56;effect on life,231;in atmosphere,19.Wave, effect on movement of water,176.Weather, changes of,31f.;origin of,174;variations,52.Wells, H. G., cited,35.Wendingstadt, storm-flood,99.Westerlies,21f.Wheat, price in England,102.White Sea, submergence,219.Whitney, J. D., cited,142.Wieland, G. R., cited,169.Williamson, E. D., cited,226.Willis, B., cited,206.Winds, at ice front,162;effect on currents,174;glaciation and,133;in Antarctica,161;in glacial period,119;in Greenland,161;planetary system of,174;velocity,240.Witch hazel, effect of light,184.Wolf, J. R., cited,61,109,288.Wolfer, cited,244.Wright, W. B., cited,35,111,119.Writing, among Mayas,96.Yucatan, Maya civilization,26,107;rainfall,108;ruins,95.Yukon, Ice Age in,221.Zante, earthquakes of,296.Zonal crowding,117.

Fig. 10. Climatic changes of 140,000 years as inferred from the stars.

According to the electro-stellar hypothesis, Alpha Centauri is more important climatically than any other star in the heavens not only because it is triple and bright, but because it is the nearest of all stars, and moves fairly rapidly. Sirius and Procyon move slowly in respect to the sun, only about eleven and eight kilometers per second respectively, and their distances at minimum arefairly large, that is, 8 and 10.2 light years. Hence their effect on the sun changes slowly. Altair moves faster, about twenty-six kilometers per second, and its minimum distance is 6.4 light years, so that its effect changes fairly rapidly. Alpha Centauri moves about twenty-four kilometers per second, and its minimum distance is only 3.2 light years. Hence its effect changes very rapidly, the change in its apparent luminosity as seen from the sun amounting at maximum to about 30 per cent in 10,000 years against 14 per cent for Altair, 4 for Sirius, and 2 for Procyon. The vast majority of the stars change so much more slowly than even Procyon that their effect is almost uniform. All the stars at a distance of more than perhaps twenty or thirty light years may be regarded as sending to the sun a practically unchanging amount of radiation. It is the bright stars within this limit which are important, and their importance increases with their proximity, their speed of motion, and the brightness and number of their companions. Hence Alpha Centauri causes the main maximum in Fig. 10, while Sirius, Altair, and Procyon combine to cause a general rise of the curve from the past to the future.

Let us now interpret Fig. 10 geologically. The low position of the curve fifty to seventy thousand years ago suggests a mild inter-glacial climate distinctly less severe than that of the present. Geologists say that such was the case. The curve suggests a glacial epoch culminating about 28,000 years ago. The best authorities put the climax of the last glacial epoch between twenty-five and thirty thousand years ago. The curve shows an amelioration of climate since that time, although it suggests that there is still considerable severity. The retreat of the ice from North America and Europe, and its persistence in Greenland and Antarctica agree with this. And the curveindicates that the change of climate is still persisting, a conclusion in harmony with the evidence as to historic changes.

If Alpha Centauri is really so important, the effect of its variations, provided it has any, ought perhaps to be evident in the sun. The activity of the star's atmosphere presumably varies, for the orbits of the two components have an eccentricity of 0.51. Hence during their period of revolution, 81.2 years, the distance between them ranges from 1,100,000,000 to 3,300,000,000 miles. They were at a minimum distance in 1388, 1459, 1550, 1631, 1713, 1794, 1875, and will be again in 1956. In Fig. 11, showing sunspot variations, it is noticeable that the years 1794 and 1875 come just at the ends of periods of unusual solar activity, as indicated by the heavy horizontal line. A similar period of great activity seems to have begun about 1914. If its duration equals the average of its two predecessors, it will end about 1950. Back in the fourteenth century a period of excessive solar activity, which has already been described, culminated from 1370 to 1385, or just before the two parts of Alpha Centauri were at a minimum distance. Thus in three and perhaps four cases the sun has been unusually active during a time when the two parts of the star were most rapidly approaching each other and when their atmospheres were presumably most disturbed and their electrical emanations strongest.

The fact that Alpha Centauri, the star which would be expected most strongly to influence the sun, and hence the earth, was nearest the sun at the climax of the last glacial epoch, and that today the solar atmosphere is most active when the star is presumably most disturbed may be of no significance. It is given for what it is worth. Its importance lies not in the fact that it proves anything, but that no contradiction is found when we test the electro-stellar hypothesis by facts which were not thought of when the hypothesis was framed. A vast amount of astronomical work is still needed before the matter can be brought to any definite conclusion. In case the hypothesis stands firm, it may be possible to use the stars as a help in determining the exact chronology of the later part of geological times. If the hypothesis is disproved, it will merely leave the question of solar variations where it is today. It will not influence the main conclusions of this book as to the causes and nature of climatic changes. Its value lies in the fact that it calls attention to new lines of research.

Fig. 11. Sunspot curve showing cycles, 1750 to 1920.

Note.The asterisks indicate two absolute minima of sunspots in 1810 and 1913, and the middle years (1780 and 1854) of two periods when the sunspot maxima never fell below 95. If Alpha Centauri has an effect on the sun's atmosphere, the end of another such period would be expected not far from 1957.

Although the problems of this book may lead far afield, they ultimately bring us back to the earth and to the present. Several times in the preceding pages there has been mention of the fact that periods of extreme climatic fluctuations are closely associated with great movements of the earth's crust whereby mountains are uplifted and continents upheaved. In attempting to explain this association the general tendency has been to look largely at the past instead of the present. Hence it has been almost impossible to choose among three possibilities, all beset with difficulties. First, the movements of the crust may have caused the climatic fluctuations; second, climatic changes may cause crustal movements; and third, variations in solar activity or in some other outside agency may give rise to both types of terrestrial phenomena.

The idea that movements of the earth's crust are the main cause of geological changes of climate is becoming increasingly untenable as the complexity and rapidity of climatic changes become more clear, especially during post-glacial times. It implies that the earth's surface moves up and down with a speed and facility which appear to be out of the question. If volcanic activity be invoked the problem becomes no clearer. Even if volcanic dust should fill the air frequently and completely, neither its presence nor absence would produce such peculiar featuresas the localization of glaciers, the distribution of loess, and the mild climate of most parts of geological time. Nevertheless, because of the great difficulties presented by the other two possibilities many geologists still hold that directly or indirectly the greater climatic changes have been mainly due to movements of the earth's crust and to the reaction of the crustal movements on the atmosphere.

The possibility that climatic changes are in themselves a cause of movements of the earth's crust seems so improbable that no one appears to have investigated it with any seriousness. Nevertheless, it is worth while to raise the question whether climatic extremes may coöperate with other agencies in setting the time when the earth's crust shall be deformed.

As to the third possibility, it is perfectly logical to ascribe both climatic changes and crustal deformation to some outside agency, solar or otherwise, but hitherto there has been so little evidence on this point that such an ascription has merely begged the question. If heavenly bodies should approach the earth closely enough so that their gravitational stresses caused crustal deformation, all life would presumably be destroyed. As to the sun, there has hitherto been no conclusive evidence that it is related to crustal movements, although various writers have made suggestions along this line. In this chapter we shall carry these suggestions further and shall see that they are at least worthy of study.

As a preliminary to this study it may be well to note that the coincidence between movements of the earth's crust and climatic changes is not so absolute as is sometimes supposed. For example, the profound crustal changes at the end of the Mesozoic were not accompanied by widespread glaciation so far as is yet known, althoughthe temperature appears to have been lowered. Nor was the violent volcanic and diastrophic activity in the Miocene associated with extreme climates. Indeed, there appears to have been little contrast from zone to zone, for figs, bread fruit trees, tree ferns, and other plants of low latitudes grew in Greenland. Nevertheless, both at the end of the Mesozoic and in the Miocene the climate may possibly have been severe for a time, although the record is lost. On the other hand, Kirk's recent discovery of glacial till in Alaska between beds carrying an undoubted Middle Silurian fauna indicates glaciation at a time when there was little movement of the crust so far as yet appears.[129]Thus we conclude that while climatic changes and crustal movements usually occur together, they may occur separately.

According to the solar-cyclonic hypothesis such a condition is to be expected. If the sun were especially active when the terrestrial conditions prohibited glaciation, changes of climate would still occur, but they would be milder than under other circumstances, and would leave little record in the rocks. Or there might be glaciation in high latitudes, such as that of southern Alaska in the Middle Silurian, and none elsewhere. On the other hand, when the sun was so inactive that no great storminess occurred, the upheaval of continents and the building of mountains might go on without the formation of ice sheets, as apparently happened at the end of the Mesozoic. The lack of absolute coincidence between glaciation and periods of widespread emergence of the lands is evident even today, for there is no reason to suppose that the lands are notably lower or less extensive now than they were during the Pleistocene glaciation. In fact, there is much evidence that many areas have risensince that time. Yet glaciation is now far less extensive than in the Pleistocene. Any attempt to explain this difference on the basis of terrestrial changes is extremely difficult, for the shape and altitude of continents and mountains have not changed much in twenty or thirty thousand years. Yet the present moderately mild epoch, like the puzzling inter-glacial epochs of earlier times, is easily explicable on the assumption that the sun's atmosphere may sometimes vary in harmony with crustal activity, but does not necessarily do so at all times.

Turning now to the main problem of how climatic changes may be connected with movements of the earth's crust, let us follow our usual method and examine what is happening today. Let us first inquire whether earthquakes, which are one of the chief evidences that crustal movements are actually taking place in our own times, show any connection with sunspots. In order to test this, we have comparedMilne's Catalogue of Destructive Earthquakesfrom 1800 to 1899, with Wolf's sunspot numbers for the same period month by month. The earthquake catalogue, as its compiler describes it, "is an attempt to give a list of earthquakes which have announced changes of geological importance in the earth's crust; movements which have probably resulted in the creation or the extension of a line of fault, the vibrations accompanying which could, with proper instruments, have been recorded over a continent or the whole surface of our world. Small earthquakes have been excluded, while the number of large earthquakes both for ancient and modern times has been extended. As an illustration of exclusion, I may mention that between 1800 and 1808, which are years taken at random, I find in Mallet's catalogue 407 entries. Only thirty-seven of these, which were accompanied by structural damage, have been retained.Other catalogues such as those of Perry and Fuchs have been treated similarly."[130]

If the earthquakes in such a carefully selected list bear a distinct relation to sunspots, it is at least possible and perhaps probable that a similar relation may exist between solar activity and geological changes in the earth's crust. The result of the comparison of earthquakes and sunspots is shown in Table 7. The first column gives the sunspot numbers; the second, the number of months that had the respective spot numbers during the century from 1800 to 1899. Column C shows the total number of earthquakes during the months having any particular degree of spottedness; while D, which is the significant column, gives the average number of destructive earthquakes per month under each of the six conditions of solar spottedness.

The regularity of column D is so great as to make it almost certain that we are here dealing with a real relationship. Column F, which shows the average number of earthquakes in the month succeeding any given condition of the sun, is still more regular except for the last entry.

The chance that six numbers taken at random will arrange themselves in any given order is one in 720. In other words, there is one chance in 720 that the regularity of column D is accidental. But column F is as regular as column D except for the last entry. If columns D and E were independent there would be one chance in about 500,000 that the six numbers in both columns would fall in the same order, and one chance in 14,400 that five numbers in each would fall in the same order. But the two columns are somewhat related, for although the after-shocks of a great earthquake are never included in Milne's table, a world-shaking earthquake in one region during a given month probably creates conditions that favor similar earthquakes elsewhere during the next month. Hence the probability that we are dealing with a purely accidental arrangement in Table 7 is less than one in 14,400 and greater than one in 500,000. It may be one in 20,000 or 100,000. In any event it is so slight that there is high probability that directly or indirectly sunspots and earthquakes are somehow connected.

In ascertaining the relation between sunspots and earthquakes it would be well if we could employ the strict method of correlation coefficients. This, however, is impossible for the entire century, for the record is by no means homogeneous. The earlier decades are represented by only about one-fourth as many earthquakes as the later ones, a condition which is presumably due to lack of information. This makes no difference with the methodemployed in Table 7, since years with many and few sunspots are distributed almost equally throughout the entire nineteenth century, but it renders the method of correlation coefficients inapplicable. During the period from 1850 onward the record is much more nearly homogeneous, though not completely so. Even in these later decades, however, allowance must be made for the fact that there are more earthquakes in winter than in summer, the average number per month for the fifty years being as follows:

The correlation coefficient between the departures from these monthly averages and the corresponding departures from the monthly averages of the sunspots for the same period, 1850-1899, are as follows:

Sunspots and earthquakes of same month: +0.042, or 1.5 times the probable error.Sunspots of a given month and earthquakes of that month and the next: +0.084, or 3.1 times the probable error.Sunspots of three consecutive months and earthquakes of three consecutive months allowing a lag of one month, i.e., sunspots of January, February, and March compared with earthquakes of February, March, and April; sunspots of February, March, and April with earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the probable error.

Sunspots and earthquakes of same month: +0.042, or 1.5 times the probable error.

Sunspots of a given month and earthquakes of that month and the next: +0.084, or 3.1 times the probable error.

Sunspots of three consecutive months and earthquakes of three consecutive months allowing a lag of one month, i.e., sunspots of January, February, and March compared with earthquakes of February, March, and April; sunspots of February, March, and April with earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the probable error.

These coefficients are all small, but the number of individual cases, 600 months, is so large that the probable error is greatly reduced, being only ±0.027 or ±0.028. Moreover, the nature of our data is such that even ifthere is a strong connection between solar changes and earth movements, we should not expect a large correlation coefficient. In the first place, as already mentioned, the earthquake data are not strictly homogeneous. Second, an average of about two and one-half strong earthquakes per month is at best only a most imperfect indication of the actual movement of the earth's crust. Third, the sunspots are only a partial and imperfect measure of the activity of the sun's atmosphere. Fourth, the relation between solar activity and earthquakes is almost certainly indirect. In view of all these conditions, the regularity of Table 7 and the fact that the most important correlation coefficient rises to more than four times the probable error makes it almost certain that the solar and terrestrial phenomena are really connected.

We are now confronted by the perplexing question of how this connection can take place. Thus far only three possibilities present themselves, and each is open to objections. The chief agencies concerned in these three possibilities are heat, electricity, and atmospheric pressure. Heat may be dismissed very briefly. We have seen that the earth's surface becomes relatively cool when the sun is active. Theoretically even the slightest change in the temperature of the earth's surface must influence the thermal gradient far into the interior and hence cause a change of volume which might cause movements of the crust. Practically the heat of the surface ceases to be of appreciable importance at a depth of perhaps twenty feet, and even at that depth it does not act quickly enough to cause the relatively prompt response which seems to be characteristic of earthquakes in respect to the sun.

The second possibility is based on the relationship between solar and terrestrial electricity. When the sun is active the earth's atmospheric electrical potential issubject to slight variations. It is well known that when two opposing points of an ionized solution are oppositely charged electrically, a current passes through the liquid and sets up electrolysis whereby there is a segregation of materials, and a consequent change in the volume of the parts near the respective electrical poles. The same process takes place, although less freely, in a hot mass such as forms the interior of the earth. The question arises whether internal electrical currents may not pass between the two oppositely charged poles of the earth, or even between the great continental masses and the regions of heavier rock which underlie the oceans. Could this lead to electrolysis, hence to differentiation in volume, and thus to movements of the earth's crust? Could the results vary in harmony with the sun? Bowie[131]has shown that numerous measurements of the strength and direction of the earth's gravitative pull are explicable only on the assumption that the upheaval of a continent or a mountain range is due in part not merely to pressure, or even to flowage of the rocks beneath the crust, but also to an actual change in volume whereby the rocks beneath the continent attain relatively great volume and those under the oceans a small volume in proportion to their weight. The query arises whether this change of volume may be related to electrical currents at some depth below the earth's surface.

The objections to this hypothesis are numerous. First, there is little evidence of electrolytic differentiation in the rocks. Second, the outer part of the earth's crust is a very poor conductor so that it is doubtful whether even a high degree of electrification of the surface would have much effect on the interior. Third, electrolysis due to anysuch mild causes as we have here postulated must be an extremely slow process, too slow, presumably, to have any appreciable result within a month or two. Other objections join with these three in making it seem improbable that the sun's electrical activity has any direct effect upon movements of the earth's crust.

The third, or meteorological hypothesis, which makes barometric pressure the main intermediary between solar activity and earthquakes, seems at first sight almost as improbable as the thermal and electrical hypotheses. Nevertheless, it has a certain degree of observational support of a kind which is wholly lacking in the other two cases. Among the extensive writings on the periodicity of earthquakes one main fact stands out with great distinctness: earthquakes vary in number according to the season. This fact has already been shown incidentally in the table of earthquake frequency by months. If allowance is made for the fact that February is a short month, there is a regular decrease in the frequency of severe earthquakes from December and January to June. Since most of Milne's earthquakes occurred in the northern hemisphere, this means that severe earthquakes occur in winter about 20 per cent oftener than in summer.

The most thorough investigation of this subject seems to have been that of Davisson.[132]His results have been worked over and amplified by Knott,[133]who has tested them by Schuster's exact mathematical methods. His results are given in Table 8.[134]Here the northern hemisphereis placed first; then come the East Indies and the Malay Archipelago lying close to the equator; and finally the southern hemisphere. In the northern hemisphere practically all the maxima come in the winter, for the month of December appears in fifteen cases out of the twenty-five in column D, while January, February, or November appears in six others. It is also noticeable that in sixteen cases out of twenty-five the ratio of the actual to the expected amplitude in column G is four or more, so that a real relationship is indicated, while the ratio falls below three only in Japan and Zante. The equatorial data, unlike those of the northern hemisphere, are indefinite, for in the East Indies no month shows a marked maximum and the expected amplitude exceeds the actual amplitude. Even in the Malay Archipelago, which shows a maximum in May, the ratio of actual to expected amplitude is only 2.6. Turning to the southern hemisphere, the winter months of that hemisphere are as strongly marked by a maximum as are the winter months of the northernhemisphere. July or August appears in five out of six cases. Here the ratio between the actual and expected amplitudes is not so great as in the northern hemisphere. Nevertheless, it is practically four in Chile, and exceeds five in Peru and Bolivia, and in the data for the entire southern hemisphere.

The whole relationship between earthquakes and the seasons in the northern and southern hemispheres is summed up in Fig. 12 taken from Knott. The northern hemisphere shows a regular diminution in earthquake frequency from December until June, and an increase the rest of the year. In the southern hemisphere the course of events is the same so far as summer and winter are concerned, for August with its maximum comes in winter, while February with its minimum comes in summer. In the southern hemisphere the winter month of greatest seismic activity has over 100 per cent more earthquakes than the summer month of least activity. In the northern hemisphere this difference is about 80 per cent, but this smaller figure occurs partly because the northern data include certain interesting and significant regions like Japan and China where the usual conditions are reversed.[135]If equatorial regions were included in Fig. 12, they would give an almost straight line.

The connection between earthquakes and the seasons is so strong that almost no students of seismology question it, although they do not agree as to its cause. A meteorological hypothesis seems to be the only logical explanation.[136]Wherever sufficient data are available, earthquakesappear to be most numerous when climatic conditions cause the earth's surface to be most heavily loaded or to change its load most rapidly. The main factor in the loading is apparently atmospheric pressure. This acts in two ways. First, when the continents become cold in winter the pressure increases. On an average the air at sea level presses upon the earth's surface at the rate of 14.7 pounds per square inch, or over a ton per square foot, and only a little short of thirty million tons per square mile. An average difference of one inch between the atmospheric pressure of summer and winter over ten million square miles of the continent of Asia, for example, means that the continent's load in winter is about ten million million tons heavier than in summer. Second, the changes in atmospheric pressure due to the passage of storms are relatively sharp and sudden. Hence they are probably more effective than the variations in the load from season to season. This is suggested by the rapidity with which the terrestrial response seems to follow the supposed solar cause of earthquakes. It is also suggested by the fact that violent storms are frequently followed by violent earthquakes. "Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in the typhoon region of Japan, China, and the East Indies. During tropical hurricanes a change of pressure amounting to half an inch in two hours is common. On September22, 1885, at False Point Lighthouse on the Bay of Bengal, the barometer fell about an inch in six hours, then nearly an inch and a half in not much over two hours, and finally rose fully two inches inside of two hours. A drop of two inches in barometric pressure means that a load of about two million tons is removedfrom each square mile of land; the corresponding rise of pressure means the addition of a similar load. Such a storm, and to a less degree every other storm, strikes a blow upon the earth's surface, first by removing millions of tons of pressure and then by putting them on again.[137]Such storms, as we have seen, are much more frequent and severe when sunspots are numerous than at other times. Moreover, as Veeder[138]long ago showed, one of the most noteworthy evidences of a connection between sunspots and the weather is a sudden increase of pressure in certain widely separated high pressure areas. In most parts of the world winter is not only the season of highest pressure and of most frequent changes of Veeder's type, but also of severest storms. Hence a meteorological hypothesis would lead to the expectation that earthquakes would occur more frequently in winter than in summer. On the Chinese coast, however, and also on the oceanic side of Japan, as well as in some more tropical regions, the chief storms come in summer in the form of typhoons. These are the places where earthquakes also are most abundant in summer. Thus, wherever we turn, storms and the related barometric changes seem to be most frequent and severe at the very times when earthquakes are also most frequent.

Fig. 12. Seasonal distribution of earthquakes. (After Davisson and Knott.)——  Northern Hemisphere.- - - - Southern Hemisphere.

Other meteorological factors, such as rain, snow, winds, and currents, probably have some effect on earthquakesthrough their ability to load the earth's crust. The coming of vegetation may also help. These agencies, however, appear to be of small importance compared with the storms. In high latitudes and in regions of abundant storminess most of these factors generally combine with barometric pressure to produce frequent changes in the load of the earth's crust, especially in winter. In low latitudes, on the other hand, there are few severe storms, and relatively little contrast in pressure and vegetation from season to season; there is no snow; and the amount of ground water changes little. With this goes the twofold fact that there is no marked seasonal distribution of earthquakes, and that except in certain local volcanic areas, earthquakes appear to be rare. In proportion to the areas concerned, for example, there is little evidence of earthquakes in equatorial Africa and South America.

The question of the reality of the connection between meteorological conditions and crustal movements is so important that every possible test should be applied. At the suggestion of Professor Schlesinger we have looked up a very ingenious line of inquiry. During the last decades of the nineteenth century, a long series of extremely accurate observations of latitude disclosed a fact which had previously been suspected but not demonstrated, namely, that the earth wabbles a little about its axis. The axis itself always points in the same direction, and since the earth slides irregularly around it the latitude of all parts of the earth keeps changing. Chandler has shown that the wabbling thus induced consists of two parts. The first is a movement in a circle with a radius of about fifteen feet which is described in approximately 430 days. This so-called Eulerian movement is a normal gyroscopic motion like the slow gyration of aspinning top. This depends on purely astronomical causes, and no terrestrial cause can stop it or eliminate it. The period appears to be constant, but there are certain puzzling irregularities. The usual amplitude of this movement, as Schlesinger[139]puts it, "is about 0".27, but twice in recent years it has jumped to 0".40. Such a change could be accounted for by supposing that the earth had received a severe blow or a series of milder blows tending in the same direction." These blows, which were originally suggested by Helmert are most interesting in view of our suggestion as to the blows struck by storms.

The second movement of the pole has a period of a year, and is roughly an ellipse whose longest radius is fourteen feet and the shortest, four feet; or, to put it technically, there is an annual term with a maximum amplitude of about 0".20. This, however, varies irregularly. The result is that the pole seems to wander over the earth's surface in the spiral fashion illustrated in Fig. 13. It was early suggested that this peculiar wandering of the pole in an annual period must be due to meteorological causes. Jeffreys[140]has investigated the matter exhaustively. He assumes certain reasonable values for the weight of air added or subtracted from different parts of the earth's surface according to the seasons. He also considers the effect of precipitation, vegetation, and polar ice, and of variations of temperature and atmospheric pressure in their relation to movements of the ocean. Then he proceeds to compare allthese with the actual wandering of the pole from 1907 to 1913. While it is as yet too early to say that any special movement of the pole was due to the specific meteorological conditions of any particular year, Jeffreys' work makes it clear that meteorological causes, especially atmospheric pressure, are sufficient to cause the observed irregular wanderings. Slight wanderings may arise from various other sources such as movements of the rocks when geological faults occur or the rush of a great wave due to a submarine earthquake. So far as known, however, all these other agencies cause insignificant displacements compared with those arising from movements of the air. This fact coupled with the mathematical certainty that meteorological phenomena must produce some wandering of the pole, has caused most astronomers to accept Jeffreys' conclusion. If we follow their example we are led to conclude that changes in atmospheric pressure and in the other meteorological conditions strike blows which sometimes shift the earthseveral feet from its normal position in respect to the axis.

Fig. 13. Wandering of the pole from 1890 to 1898.(After Moulton.)

If the foregoing reasoning is correct, the great and especially the sudden departures from the smooth gyroscopic circle described by the pole in the Eulerian motion would be expected to occur at about the same time as unusual earthquake activity. This brings us to an interesting inquiry carried out by Milne[141]and amplified by Knott.[142]Taking Albrecht's representation of the irregular spiral-like motion of the pole, as given in Fig. 13, they show that there is a preponderance of severe earthquakes at times when the direction of motion of the earth in reference to its axis departs from the smooth Eulerian curve. A summary of their results is given in Table 9. The table indicates that during the period from 1892 to 1905 there were nine different times when the curve of Fig. 13 changed its direction or was deflected by less than 10° during a tenth of a year. In other words, during those periods it did not curve as much as it ought according to the Eulerian movement. At such times there were 179 world-shaking earthquakes, or an average of about 19.9 per tenth of a year. According to the other lines of Table 9, in thirty-two cases the deflection during a tenth of a year was between 10° and 25°, while in fifty-six cases it was from 25° to 40°. During these periods the curve remained close to the Eulerian path and the world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the deflection was high, that is, when meteorological conditions threw the earth far out of its Eulerian course, the earthquakes were again numerous, the number rising to 23.4 when the deflection amounted to more than 55°.

In order to test this conclusion in another way we have followed a suggestion of Professor Schlesinger. Under his advice the Eulerian motion has been eliminated and a new series of earthquake records has been compared with the remaining motions of the poles which presumably arise largely from meteorological causes. For this purpose use has been made of the very full records of earthquakes published under the auspices of the International Seismological Commission for the years 1903 to 1908, the only years for which they are available. These include every known shock of every description which was either recorded by seismographs or by direct observation in any part of the world. Each shock is given the same weight, no matter what its violence or how closely it follows another. The angle of deflection has been measured as Milne measured it, but since the Eulerian motion is eliminated, our zero is approximately the normal condition which would prevail if there were no meteorological complications. Dividing the deflections into six equal groups according to the size of the angle, we get the result shown in Table 10.

Here where some twenty thousand earthquakes are employed the result agrees closely with that of Milne for a different series of years and for a much smaller number of earthquakes. So long as the path of the pole departs less than about 45° from the smooth gyroscopic Eulerian path, the number of earthquakes is almost constant, about eight and a quarter per day. When the angle becomes large, however, the number increases by nearly 50 per cent. Thus the work of Milne, Knott, and Jeffreys is confirmed by a new investigation. Apparently earthquakes and crustal movements are somehow related to sudden changes in the load imposed on the earth's crust by meteorological conditions.

This conclusion is quite as surprising to the authors as to the reader—perhaps more so. At the beginning of this investigation we had no faith whatever in any importantrelation between climate and earthquakes. At its end we are inclined to believe that the relation is close and important.

It must not be supposed, however, that meteorological conditions are thecauseof earthquakes and of movements of the earth's crust. Even though the load that the climatic agencies can impose upon the earth's crust runs into millions of tons per square mile, it is a trifle compared with what the crust is able to support. There is, however, a great difference between the cause and the occasion of a phenomenon. Suppose that a thick sheet of glass is placed under an increasing strain. If the strain is applied slowly enough, even so rigid a material as glass will ultimately bend rather than break. But suppose that while the tension is high the glass is tapped. A gentle tap may be followed by a tiny crack. A series of little taps may be the signal for small cracks to spread in every direction. A few slightly harder taps may cause the whole sheet to break suddenly into many pieces. Yet even the hardest tap may be the merest trifle compared with the strong force which is keeping the glass in a state of strain and which would ultimately bend it if given time.

The earth as a whole appears to stand between steel and glass in rigidity. It is a matter of common observation that rocks stand high in this respect and in the consequent difficulty with which they can be bent without breaking. Because of the earth's contraction the crust endures a constant strain, which must gradually become enormous. This strain is increased by the fact that sediment is transferred from the lands to the borders of the sea and there forms areas of thick accumulation. From this has arisen the doctrine of isostasy, or of the equalization of crustal pressure. An important illustration ofthis is the oceanward and equatorial creep which has been described in Chapter XI. There we saw that when the lands have once been raised to high levels or when a shortening of the earth's axis by contraction has increased the oceanic bulge at the equator, or when the reverse has happened because of tidal retardation, the outer part of the earth appears to creep slowly back toward a position of perfect isostatic adjustment. If the sun had no influence upon the earth, either direct or indirect, isostasy and other terrestrial processes might flex the earth's crust so gradually that changes in the form and height of the lands would always take place slowly, even from the geological point of view. Thus erosion would usually be able to remove the rocks as rapidly as they were domed above the general level. If this happened, mountains would be rare or unknown, and hence climatic contrasts would be far less marked than is actually the case on our earth where crustal movements have repeatedly been rapid enough to produce mountains.

Nature's methods rarely allow so gradual an adjustment to the forces of isostasy. While the crust is under a strain, not only because of contraction, but because of changes in its load through the transference of sediments and the slow increase or decrease in the bulge at the equator, the atmosphere more or less persistently carries on the tapping process. The violence of that process varies greatly, and the variations depend largely on the severity of the climatic contrasts. If the main outlines of the cyclonic hypothesis are reliable, one of the first effects of a disturbance of the sun's atmosphere is increased storminess upon the earth. This is accompanied by increased intensity in almost every meteorological process. The most important effect, however, so far as the earth's crust is concerned would apparently be the rapid andintense changes of atmospheric pressure which would arise from the swift passage of one severe storm after another. Each storm would be a little tap on the tensely strained crust. Any single tap might be of little consequence, even though it involved a change of a billion tons in the pressure on an area no larger than the state of Rhode Island. Yet a rapid and irregular succession of such taps might possibly cause the crust to crack, and finally to collapse in response to stresses arising from the shrinkage of the earth.

Another and perhaps more important effect of variations in storminess and especially in the location of the stormy areas would be an acceleration of erosion in some places and a retardation elsewhere. A great increase in rainfall may almost denude the slopes of soil, while a diminution to the point where much of the vegetation dies off has a similar effect. If such changes should take place rapidly, great thicknesses of sediment might be concentrated in certain areas in a short time, thus disturbing the isostatic adjustment of the earth's crust. This might set up a state of strain which would ultimately have to be relieved, thus perhaps initiating profound crustal movements. Changes in the load of the earth's crust due to erosion and the deposition of sediment, no matter how rapid they may be from the geological standpoint, are slow compared with those due to changes in barometric pressure. A drop of an inch in barometric pressure is equivalent to the removal of about five inches of solid rock. Even under the most favorable circumstances, the removal of an average depth of five inches of rock or its equivalent in soil over millions of square miles would probably take several hundred years, while the removal of a similar load of air might occur in half a day or even a few hours. Thus the erosion and depositiondue to climatic variations presumably play their part in crustal deformation chiefly by producing crustal stresses, while the storms, as it were, strike sharp, sudden blows.

Suppose now that a prolonged period of world-wide mild climate, such as is described in Chapter X, should permit an enormous accumulation of stresses due to contraction and tidal retardation. Suppose that then a sudden change of climate should produce a rapid shifting of the deep soil that had accumulated on the lands, with a corresponding localization and increase in strains. Suppose also that frequent and severe storms play their part, whether great or small, by producing an intensive tapping of the crust. In such a case the ultimate collapse would be correspondingly great, as would be evident in the succeeding geological epoch. The sea floor might sink lower, the continents might be elevated, and mountain ranges might be shoved up along lines of special weakness. This is the story of the geological period as known to historical geology. The force that causes such movements would be the pull of gravity upon the crust surrounding the earth's shrinking interior. Nevertheless climatic changes might occasionally set the date when the gravitative pull would finally overcome inertia, and thus usher in the crustal movements that close old geologic periods and inaugurate new ones. This, however, could occur only if the crust were under sufficient strain. As Lawson[143]says in his discussion of the "elastic rebound theory," the sudden shifts of the crust which seem to be the underlying cause of earthquakes "can occur only after the accumulation of strain to a limit and ... this accumulation involves a slow creep of the region affected.In the long periods between great earthquakes the energy necessary for such shocks is being stored up in the rocks as elastic compression."

If a period of intense storminess should occur when the earth as a whole was in such a state of strain, the sudden release of the strains might lead to terrestrial changes which would alter the climate still further, making it more extreme, and perhaps permitting the storminess due to the solar disturbances to bring about glaciation. At the same time if volcanic activity should increase it would add its quota to the tendency toward glaciation. Nevertheless, it might easily happen that a very considerable amount of crustal movement would take place without causing a continental ice sheet or even a marked alpine ice sheet. Or again, if the strains in the earth's crust had already been largely released through other agencies before the stormy period began, the climate might become severe enough to cause glaciation in high latitudes without leading to any very marked movements of the earth's crust, as apparently happened in the Mid-Silurian period.

Here we must bring this study of the earth's evolution to a close. Its fundamental principle has been that the present, if rightly understood, affords a full key to the past. With this as a guide we have touched on many hypotheses, some essential and some unessential to the general line of thought. The first main hypothesis is that the earth's present climatic variations are correlated with changes in the solar atmosphere. This is the keynote of the whole book. It is so well established, however,that it ranks as a theory rather than as an hypothesis. Next comes the hypothesis that variations in the solar atmosphere influence the earth's climate chiefly by causing variations not only in temperature but also in atmospheric pressure and thus in storminess, wind, and rainfall. This, too, is one of the essential foundations on which the rest of the book is built, but though this cyclonic hypothesis is still a matter of discussion, it seems to be based on strong evidence. These two hypotheses might lead us astray were they not balanced by another. This other is that many climatic conditions are due to purely terrestrial causes, such as the form and altitude of the lands, the degree to which the continents are united, the movement of ocean currents, the activity of volcanoes, and the composition of the atmosphere and the ocean. Only by combining the solar and the terrestrial can the truth be perceived. Finally, the last main hypothesis of this book holds that if the climatic conditions which now prevail at times of solar activity were magnified sufficiently and if they occurred in conjunction with certain important terrestrial conditions of which there is good evidence, they would produce most of the notable phenomena of glacial periods. For example, they would explain such puzzling conditions as the localization and periodicity of glaciation, the formation of loess, and the occurrence of glaciation in low latitudes during Permian and Proterozoic times. The converse of this is that if the conditions which now prevail at times when the sun is relatively inactive should be intensified, that is, if the sun's atmosphere should become calmer than now, and if the proper terrestrial conditions of topographic form and atmospheric composition should prevail, there would arise the mild climatic conditions which appear to have prevailed during the greater part of geologicaltime. In short, there seems thus far to be no phase of the climate of the past which is not in harmony with an hypothesis which combines into a single unit the three main hypotheses of this book, solar, cyclonic, and terrestrial.

Outside the main line of thought lie several other hypotheses. Several of these, as well as some of the main hypotheses, are discussed chiefly inEarth and Sun, but as they are given a practical application in this book they deserve a place in this final summary. Each of these secondary hypotheses is in its way important. Yet any or all may prove untrue without altering our main conclusions. This point cannot be too strongly emphasized, for there is always danger that differences of opinion as to minor hypotheses and even as to details may divert attention from the main point. Among the non-essential hypotheses is the idea that the sun's atmosphere influences that of the earth electrically as well as thermally. This idea is still so new that it has only just entered the stage of active discussion, and naturally the weight of opinion is against it. Although not necessary to the main purpose of this book, it plays a minor rôle in the chapter dealing with the relation of the sun to other astronomical bodies. It also has a vital bearing on the further advance of the science of meteorology and the art of weather forecasting. Another secondary hypothesis holds that sunspots are set in motion by the planets. Whether the effect is gravitational or more probably electrical, or perhaps of some other sort, does not concern us at present, although the weight of evidence seems to point toward electronic emissions. This question, like that of the relative parts played by heat and electricity in terrestrial climatic changes, can be set aside for the moment. What does concern us is a third hypothesis, namely, thatif the planets really determine the periodicity of sunspots, even though not supplying the energy, the sun in its flight through space must have been repeatedly and more strongly influenced in the same way by many other heavenly bodies. In that case, climatic changes like those of the present, but sometimes greatly magnified, have presumably arisen because of the constantly changing position of the solar system in respect to other parts of the universe. Finally, the fourth of our secondary hypotheses postulates that at present the date of movements of the earth's crust is often determined by the fact that storms and other meteorological conditions keep changing the load upon first one part of the earth's surface and then upon another. Thus stresses that have accumulated in the earth's isostatic shell during the preceding months are released. In somewhat the same way epochs of extreme storminess and rapid erosion in the past may possibly have set the date for great movements of the earth's crust. This hypothesis, like the other three in our secondary or non-essential group, is still so new that only the first steps have been taken in testing it. Yet it seems to deserve careful study.

In testing all the hypotheses here discussed, primary and secondary alike, the first necessity is a far greater amount of quantitative work. In this book there has been a constant attempt to subject every hypothesis to the test of statistical facts of observation. Nevertheless, we have been breaking so much new ground that in many cases exact facts are not yet available, while in others they can be properly investigated only by specialists in physics, astronomy, or mathematics. In most cases the next great step is to ascertain whether the forces here called upon are actually great enough to produce the observed results. Even though they act only as a meansof releasing the far greater forces due to the contraction of the earth and the sun, they need to be rigidly tested as to their ability to play even this minor rôle. Still another line of study that cries aloud for research is a fuller comparison between earthquakes on the one hand and meteorological conditions and the wandering of the poles on the other. Finally, an extremely interesting and hopeful quest is the determination of the positions and movements of additional stars and other celestial bodies, the faint and invisible as well as the bright, in order to ascertain the probable magnitude of their influence upon the sun and thus upon the earth at various times in the past and in the future. Perhaps we are even now approaching some star that will some day give rise to a period of climatic stress like that of the fourteenth century, or possibly to a glacial epoch. Or perhaps the variations in others of the nearer stars as well as Alpha Centauri may show a close relation to changes in the sun.

Throughout this volume we have endeavored to discover new truth concerning the physical environment that has molded the evolution of all life. We have seen how delicate is the balance among the forces of nature, even though they be of the most stupendous magnitude. We have seen that a disturbance of this balance in one of the heavenly bodies may lead to profound changes in another far away. Yet during the billion years, more or less, of which we have knowledge, there appears never to have been a complete cataclysm involving the destruction of all life. One star after another, if our hypothesis is correct, has approached the solar system closely enough to set the atmosphere of the sun in such commotion that great changes of climate have occurred upon the earth. Yet never has the solar system passed so close to any other body or changed in any other way sufficientlyto blot out all living things. The effect of climatic changes has always been to alter the environment and therefore to destroy part of the life of a given time, but with this there has invariably gone a stimulus to other organic types. New adaptations have occurred, new lines of evolutionary progress have been initiated, and the net result has been greater organic diversity and richness. Temporarily a great change of climate may seem to retard evolution, but only for a moment as the geologist counts time. Then it becomes evident that the march of progress has actually been more rapid than usual. Thus the main periods of climatic stress are the most conspicuous milestones upon the upward path toward more varied adaptation. The end of each such period of stress has found the life of the world nearer to the high mentality which reaches out to the utmost limits of space, of time, and of thought in the search for some explanation of the meaning of the universe. Each approach of the sun to other bodies, if such be the cause of the major climatic changes, has brought the organic world one step nearer to the solution of the greatest of all problems,—the problem of whether there is a psychic goal beyond the mental goal toward which we are moving with ever accelerating speed. Throughout the vast eons of geological time the adjustment of force to force, of one body of matter to another, and of the physical environment to the organic response has been so delicate, and has tended so steadily toward the one main line of mental progress that there seems to be a purpose in it all. If the cosmic uniformity of climate continues to prevail and if the uniformity is varied by changes as stimulating as those of the past, the imagination can scarcely picture the wonders of the future. In the course of millions or even billions of years the development of mind, and perhaps of soul, many excelthat of today as far as the highest known type of mentality excels the primitive plasma from which all life appears to have arisen.

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