Plate XXXXXX.—Deluge of the North of Europe.
XXX.—Deluge of the North of Europe.
The physical proof of thisdeluge of the north of Europeexists in the accumulation of unstratified deposits which covers all the plains and low grounds of Northern Europe. On and in this deposit are found numerous blocks which have received the characteristic and significant name of erratic blocks, and which are frequently of considerable size. These become more characteristic as we ascend to higher latitudes, as in Norway, Sweden, and Denmark, the southern borders of the Baltic, and in the British Islands generally, in all of which countries deposits of marine fossil shells occur, which prove the submergence of large areas of Scandinavia, of the British Isles, and other regions during parts of the glacial period. Some of these rocks, characterised aserratic, are of very considerable volume; such, for instance, is the granite block which forms the pedestal of the statue of Peter the Great at St. Petersburg. This block was found in the interior of Russia, where the whole formation isPermian, and its presence there can only be explained by supposing it to have been transported by some vast iceberg, carried by a diluvial current. This hypothesis alone enables us to account for another block of granite, weighing about 340 tons, which was found on the sandy plains in the north of Prussia, an immense model of which was made for the Berlin Museum. The last of these erratic blocks deposited in Germany covers the grave of King Gustavus Adolphus, of Sweden, killed at the battle of Lutzen, in 1632. He was interred beneath therock. Another similar block has been raised in Germany into a monument to the geologist Leopold von Buch.
These erratic blocks which are met with in the plains of Russia, Poland, and Prussia, and in the eastern parts of England, are composed of rocks entirely foreign to the region where they are found. They belong to the primary rocks of Norway; they have been transported to their present sites, protected by a covering of ice, by the waters of the northern deluge. How vast must have been the impulsive force which could carry such enormous masses across the Baltic, and so far inland as the places where they have been deposited for the surprise of the geologist or the contemplation of the thoughtful!
The second European deluge is supposed to have been the result of the formation and upheaval of the Alps. It has filled with débris and transported material the valleys of France, Germany, and Italy over a circumference which has the Alps for its centre. The proofs of a great convulsion at a comparatively recent geological date are numerous. The Alps may be from eighty to 100 miles across, and the probabilities are that their existence is due, as Sir Charles Lyell supposes, to a succession of unequal movements of upheaval and subsidence; that the Alpine region had been exposed for countless ages to the action of rain and rivers, and that the larger valleys were of pre-glacial times, is highly probable. In the eastern part of the chain some of the Primary fossiliferous rocks, as well as Oolitic and Cretaceous rocks, and even Tertiary deposits, are observable; but in the central Alps these disappear, and more recent rocks, in some places even Eocene strata, graduate into metamorphic rocks, in which Oolitic, Cretaceous, and Eocene strata have been altered into granular marble, gneiss, and other metamorphic schists; showing that eruptions continued after the deposit of the Middle Eocene formations. Again, in the Swiss and Savoy Alps, Oolitic and Cretaceous formations have been elevated to the height of 12,000 feet, and Eocene strata 10,000 feet above the level of the sea; while in the Rothal, in the Bernese Alps, occurs a mass of gneiss 1,000 feet thick between two strata containing Oolitic fossils.
Besides these proofs of recent upheaval, we can trace effects of two different kinds, resulting from the powerful action of masses of water violently displaced by this gigantic upheaval. At first broad tracks have been hollowed out by the diluvial waves, which have, at these points, formed deep valleys. Afterwards these valleys have been filled up by materials derived from the mountain and transported into the valley, these materials consisting of rounded pebbles, argillaceousand sandy mud, generally calcareous and ferriferous. This double effect is exhibited, with more or less distinctness, in all the great valleys of the centre and south of France. The valley of the Garonne is, in respect to these phenomena, classic ground, as it were.
As we leave the little city of Muret, three successive levels will be observed on the left bank of the Garonne. The lowest of the three is that of the valley, properly so called; while the loftiest corresponds to the plateau of Saint-Gaudens. These three levels are distinctly marked in the Toulousean country, which illustrates the diluvial phenomena in a remarkable fashion. The city of Toulouse reposes upon a slight eminence of diluvial formation. The flat diluvial plateau contrasts strongly with the rounded hills of Gascony and Languedoc. They are essentially constituted of a bed of gravel, formed of rounded or oval pebbles, and again covered with sandy and earthy deposits. The pebbles are principally quartzose, brown or black externally, mixed with portions of hard “Old Red” and New Red Sandstone. The soft earth which accompanies the pebbles and gravel is a mixture of argillaceous sand of a red or yellow colour, caused by the oxide of iron which enters into its composition. In the valley, properly so called, we find the pebbles again associated with other minerals which are rare at the higher levels. Some teeth of the Mammoth, andRhinoceros tichorhinus, have been found at several points on the borders of this valley.
The small valleys, tributary to the principal valley, would appear to have been excavated secondarily, partly out of diluvial deposits, and their alluvium, essentially earthy, has been formed at the expense of the Tertiary formation, and even of the diluvium itself. Among other celebrated sites, the diluvial formation is largely developed in Sicily. The ancient temple of the Parthenon at Athens is built on an eminence formed of diluvial earth.
In the valley of the Rhine, in Alsace, and in many isolated parts of Europe, a particular sort ofdiluviumforms thick beds; it consists of a yellowish-grey mud, composed of argillaceous matter mixed with carbonate of lime, quartzose and micaceous sand, and oxide of iron. This mud, termed by geologistsloess, attains in some places considerable thickness. It is recognisable in the neighbourhood of Paris. It rises a little both on the right and left, above the base of the mountains of the Black Forest and of the Vosges; and forms thick beds on the banks of the Rhine.
The fossils contained in diluvial deposits consist, generally, of terrestrial, lacustrine, or fluviatile shells, for the most part belongingto species still living. In parts of the valley of the Rhine, between Bingen and Basle, the fluviatile loam or loess, now under consideration, is seen forming hills several hundred feet thick, and containing, here and there, throughout that thickness, land and fresh-water shells; from which it seems necessary to suppose, according to Lyell, first, a time when the loess was slowly accumulated, then a later period, when large portions of it were removed—and followed by movements of oscillation, consisting, first, of a general depression, and then of a gradual re-elevation of the land.
We have already noticed the caverns in which such extraordinary accumulations of animal remains were discovered: it will not be out of place to give here a résumé of the state of our knowledge concerningbone-cavesandbone-breccias.
Thebone-cavesare not simply cavities hollowed out of the rock; they generally consist of numerous chambers or caverns communicating with each other by narrow passages (often of considerable length) which can only be traversed by creeping. One in Mexico extends several leagues. Perhaps the most remarkable in Europe is that of Gailenreuth in Franconia. The Harz mountains contain many fine caverns; among others, those of Scharrfeld andBaumann’s Hohl, in which many bones of Hyæna, Bears, and Lions have been found together. TheKirkdale Cave, so well known from the description given of it by Dr. Buckland, lying about twenty-five miles north-north-east of York, was the burial-place, as we have stated, of at least 300 Hyænas belonging to individuals of different ages; besides containing some other remains, mostly teeth (those of the Hyæna excepted) belonging to ruminating animals. Buckland states that the bones of all the other animals, those of the Hyænas not excepted, were gnawed. He also noticed a partial polish and wearing away to a considerable depth of one side of many of the best preserved specimens of teeth and bones, which can only be accounted for by referring the partial destruction to the continual treading of the Hyænas, and the rubbing of their skin on the side that lay uppermost at the bottom of the den.
From these facts it would appear probable that the Cave at Kirkdale was, “during a long succession of years, inhabited as a den by Hyænas, and that they dragged into its recesses the other animal bodies, whose remains are found mixed indiscriminately with their own.”[103]This conjecture is made almost certain by the discoverymade by Dr. Buckland of many coprolites of animals that had fed on bones, as well as traces of the frequent passage of these animals to or from the entrance of the cavern or den. A modern naturalist visiting the Cavern of Adelsberg, in Carniola, traversed a series of chambers extending over three leagues in the same direction, and was only stopped in his subterranean discoveries by coming to a lake which occupied its entire breadth.
The interior walls of the bone-caves are, in general, rounded off, and furrowed, presenting many traces of the erosive action of water, characteristics which frequently escape observation because the walls are covered with the calcareous deposit calledstalactiteorstalagmite—that is, with carbonate of lime, resulting from the deposition left by infiltrating water, through the overlying limestone, into the interior of the cavern. The formation of the stalactite, with which many of the bones were incrusted in the Cave of Gailenreuth, is thus described by Liebig. The limestone over the cavern is covered with a rich soil, in which the vegetable matter is continually decaying. This mould, or humus, being acted on by moisture and air, evolves carbonic acid, which is dissolved by rain. The rain-water thus impregnated, permeating the porous limestone, dissolves a portion of it, and afterwards, when the excess of carbonic acid evaporates in the caverns, parts with the calcareous matter, and formsstalactite—the stalactites being the pendent masses of carbonate of lime, which hang in picturesque forms either in continuous sheets, giving the cave and its sides the appearance of being hung with drapery, or like icicles suspended from the roof of the cave, through which the water percolates; while those formed on the surface of the floor formstalagmite. These calcareous products ornament the walls of these gloomy caverns in a most brilliant and picturesque manner.
Under a covering of stalagmite, the floor of the cave frequently presents deposits of mud and gravel. It is in excavating this soil that the bones of antediluvian animals, mixed with shells, fragments of rocks, and rolled pebbles, are discovered. The distribution of these bones in the middle of the gravelly argillaceous mud is as irregular as possible. The skeletons are rarely entire; the bones do not even occur in their natural positions. The bones of small Rodents are found accumulated in the crania of great Carnivora. The teeth of Bears, Hyænas, and Rhinoceros are cemented with the jaw-bones of Ruminants. The bones are very often polished and rounded, as if they had been transported from great distances; others are fissured; others, nevertheless, are scarcely altered. Their state of preservation varies with their position in the cave.
The bones most frequently found in caves are those of the Carnivora of the Quaternary epoch: the Bear, Hyæna, the Lion, and Tiger. The animals of the plain, and notably the great Pachyderms—the Mammoth and Rhinoceros—are only very rarely met with, and always in small numbers. From the cavern of Gailenreuth more than a thousand skeletons have been taken, of which 800 belonged to the largeUrsus spelæus, and sixty to the smaller species, with 200 Hyænas, Wolves, Lions, and Gluttons. A jaw of the Glutton has lately been found by Mr. T. McK. Hughes in a cave in the Mountain Limestone at Plas Heaton, associated with Wolf, Bison, Reindeer, Horse, and Cave Bear; proving that the Glutton, which at the present day inhabits Siberia and the inclement northern regions of Europe, inhabited Great Britain during the Pleistocene or Quaternary Period. In the Kirkdale cave the remains, as we have seen, included those of not less than 300 Hyænas of all ages. Dr. Buckland concludes, from these circumstances, that the Hyænas alone made this their den, and that the bones of other animals accumulated there had been carried thither by them as their prey; it is, however, now admitted that this part of the English geologist’s conclusions do not apply to the contents of all bone-caves. In some instances the bones of the Mammals are broken and worn as with a long transport,rolled, according to the technical geological expression, and finally cemented in the same mud, together with fragments of the rocks of the neighbourhood. Besides bones of Hyænas, are found not only the bones of inoffensive herbivora, but remains of Lions and Bears.
We ought to note, in order to make this explanation complete, that some geologists consider that these caves served as a refuge for sick and wounded animals. It is certain that we see, in our own days, some animals, when attacked by sickness, seek refuge in the fissures of rocks, or in the hollows of trunks of trees, where they die; to this natural impulse it may, probably, be ascribed that the skeletons of animals are so rarely found in forests or plains. We may conclude, then, that besides the more general mode in which these caverns were filled with bones, the two other causes which we have enumerated may have been in operation; that is to say, they were the habitual sojourn of carnivorous and destructive animals, and they became the retreat of sick animals on some particular occasions.
What was the origin of these caves? How have these immense excavations been produced? Nearly all these caves occur in limestone rocks, particularly in the Jurassic and Carboniferous formations, which present many vast subterranean caverns. At the sametime some fine caves exist in the Silurian formation, such as theGrotto des Demoiselles(Fig. 194) near Ganges, of Hérault. It should be added, in order to complete the explanation of the cave formations, that the greater part of these vast internal excavations have been chiefly caused by subterranean watercourses, which have eroded and washed away a portion of the walls, and in this manner greatly enlarged their original dimensions.
But there are other modes than the above of accounting, in a more satisfactory manner, for the existence of these caves. According to Sir Charles Lyell, there was a time when (as now) limestone rocks were dissolved, and when the carbonate of lime was carried away gradually by springs from the interior of the earth; that another era occurred, when engulfed rivers or occasional floods swept organic and inorganic débris into the subterranean hollows previously formed; finally, there were changes, in which engulfed rivers were turned into new channels, and springs dried up, after which the cave-mud, breccia, gravel, and fossil bones were left in the position in which they are now discovered. “We know,” says that eminent geologist,[104]“that in every limestone district the rain-water issoft, or free from earthy ingredients, when it falls upon the soil, and when it enters the rocks below; whereas it ishard, or charged with carbonate of lime, when it issues again to the surface in springs. The rain derives some of its carbonic acid from the air, but more from the decay of vegetable matter in the soil through which it percolates; and by the excess of this acid, limestone is dissolved, and the water becomes charged with carbonate of lime. The mass of solid matter silently and unceasingly subtracted in this way from the rocks in every century is considerable, and must in the course of thousands of years be so vast, that the space it once occupied may well be expressed by a long suite of caverns.”
The most celebrated of these bone-caves are those of Gailenreuth, in Franconia; of Nabenstein, and of Brumberg, in the same country; the caves on the banks of the Meuse, near Liège, of which the late Dr. Schmerling examined forty; of Yorkshire, Devonshire, Somersetshire, and Derbyshire, in England; also several in Sicily, at Palermo, and Syracuse; in France at Hérault, in the Cévennes, and Franche Comté; and in the New World, in Kentucky and Virginia.
Theossiferous brecciadiffers from the bone-caves only in form. The most remarkable of them are seen at Cette, Antibes, and Nice, on the shores of Italy; and in the isles of Corsica, Malta, and Sardinia.
Fig. 194Fig. 194.—Grotto des Demoiselles, Hérault.
Fig. 194.—Grotto des Demoiselles, Hérault.
Nearly the same bones are found in thebrecciawhich we find inthe caves; the chief difference being that fossils of the Ruminants are there in greater abundance. The proportions of bones to the fragments of stone and cement vary considerably in different localities. In thebrecciaof Cagliari, where the remains of Ruminants are less abundant than at Gibraltar and Nice, the bones, which are those of the small Rodents, are, so to speak, more abundant than the mud in which they are embedded. We find, there, also, three or four species of Birds which belong to Thrushes and Larks. In thebrecciaatNice the remains of some great Carnivora are found, among which are recognised two species of Lion and Panther. In the Grotto di San-Ciro, in the Monte Griffone, about six miles from Palermo, in Sicily, Dr. Falconer collected remains of two species of Hippopotamus and bones ofElephas antiquus, Bos, Stag, Pig, Bear, Dog, and a largeFelis, some of which indicated a Pliocene age. Like many others, this cave contains a thick mass of bone-breccia on its floor, the bones of which have long been known, and were formerly supposed to be those of giants; while Prof. Ferrara suggested that the Elephants’ bones were due to the Carthaginian elephants imported into Sicily for purposes of sport.[105]
But thebrecciais not confined to Europe. We meet with it in all parts of the globe; and recent discoveries in Australia indicate a formation corresponding exactly to theossiferous brecciaof the Mediterranean, in which an ochreous-reddish cement binds together fragments of rocks and bones, among which we find four species of Kangaroos.
Fig. 195Fig. 195.—Beloptera Sepioidea.
Fig. 195.—Beloptera Sepioidea.
The two cataclysms, of which we have spoken, surprised Europe at the moment of the development of an important creation. The whole scope of animated Nature, the evolution of animals, was suddenly arrested in that part of our hemisphere over which these gigantic convulsions spread, followed by the brief but sudden submersion of entire continents. Organic life had scarcely recovered from the violent shock, when a second, and perhaps severer blow assailed it. The northern and central parts of Europe, the vast countries which extend from Scandinavia to the Mediterranean and the Danube, were visited by a period of sudden and severe cold: the temperature of the polar regions seized them. The plains of Europe, but now ornamented by the luxurious vegetation developed by the heat of a burning climate, the boundless pastures on which herds of great Elephants, the active Horse, the robust Hippopotamus, and great Carnivorous animals grazed and roamed, became covered with a mantle of ice and snow.
To what cause are we to attribute a phenomenon so unforeseen, and exercising itself with such intensity? In the present state of our knowledge no certain explanation of the event can be given. Did the central planet, the sun, which was long supposed to distribute light and heat to the earth, lose during this period its calorific powers? This explanation is insufficient, since at this period the solar heat is not supposed to have greatly influenced the earth’s temperature. Were the marine currents, such as theGulf Stream, which carries the Atlantic Ocean towards the north and west of Europe, warming and raising its temperature, suddenly turned in the contrary direction? No such hypothesis is sufficient to explain either the cataclysms or the glacial phenomena; and we need not hesitate to confess our ignorance of this strange, this mysterious, episode in the history of the globe.
There have been attempts, and very ingenious ones too, to explain these phenomena, of which we shall give a brief summary,without committing ourselves to any further opinion, using for that purpose the information contained in M. Ch. Martins’ excellent work. “The most violent convulsions of the solid and liquid elements,” says this able writer, “appear to have been themselves only the effects due to a cause much more powerful than the mere expansion of the pyrosphere; and it is necessary to recur, in order to explain them, to some new and bolder hypothesis than has yet been hazarded. Some philosophers have belief in an astronomical revolution which may have overtaken our globe in the first age of its formation, and have modified its position in relation to the sun. They admit that the poles have not always been as they are now, and that some terrible shock displaced them, changing at the same time the inclination of the axis of the rotation of the earth.” This hypothesis, which is nearly the same as that propounded by the Danish geologist, Klee, has been ably developed by M. de Boucheporn. According to this writer, many multiplied shocks, caused by the violent contact of the earth with comets, produced the elevation of mountains, the displacement of seas, and perturbations of climate—phenomena which he ascribes to the sudden disturbance of the parallelism of the axis of rotation. The antediluvian equator, according to him, makes a right angle with the existing equator.
“Quite recently,” adds M. Martins, “a learned French mathematician, M. J. Adhémar, has taken up the same idea; but, dismissing the more problematical elements of the concussion with comets as untenable, he seeks to explain the deluges by the laws of gravitation and celestial mechanics, and his theory has been supported by very competent writers. It is this: We know that our planet is influenced by two essential movements—one of rotation on its axis, which it accomplishes in twenty-four hours; the other of translation, which it accomplishes in a little more than 3651⁄4days. But besides these great and perceptible movements, the earth has a third, and even a fourth movement, with one of which we need not occupy ourselves; it is that designatednutationby astronomers. It changes periodically, but within very restricted limits, the inclination of the terrestrial axis to the plane of the ecliptic by a slight oscillation, the duration of which is only eighteen hours, and its influence upon the relative length of day and night almost inappreciable. The other movement is that on which M. Adhémar’s theory is founded.
“We know that the curve described by the earth in its annual revolution round the sun is not a circle, but an ellipse; that is, a slightly elongated circle, sometimes called a circle of two centres, one of which is occupied by the sun. This curve is called the ecliptic.We know, also, that, in its movement of translation, the earth preserves such a position that its axis of rotation is intercepted, at its centre, by the plane of the ecliptic. But in place of being perpendicular, or at right angles with this plane, it crosses it obliquely in such a manner as to form on one side an angle of one-fourth, and on the other an angle of three-fourths of a right angle. This inclination is only altered in an insignificant degree by the movement ofnutation. I need scarcely add that the earth, in its annual revolution, occupies periodically four principal positions on the ecliptic, which mark the limits of the four seasons. When its centre is at the extremity most remote from the sun, oraphelion, it is the summer solstice for the northern hemisphere. When its centre is at the other extremity, orperihelion, the same hemisphere is at the winter solstice. The two intermediate points mark the equinoxes of spring and autumn. The great circle of separation of light and shade passes, then, precisely through the poles, the day and night are equal, and the line of intersection of the plane of the equator and that of the ecliptic make part of the vector ray from the centre of the sun to the centre of the earth—what we call theequinoctial line.
“Thus placed, it is evident that if the terrestrial axis remained always parallel to itself, the equinoctial line would always pass through the same point on the surface of the globe. But it is not absolutely thus. The parallelism of the axis of the earth is changed slowly, very slowly, by a movement which Arago ingeniously compares to the varying inclination of a top when about to cease spinning. This movement has the effect of making the equinoctial points on the surface of the earth retrograde towards the east from year to year, in such a manner that at the end of 25,800 years according to some astronomers, but 21,000 years according to Adhémar, the equinoctial point has literally made a circuit of the globe, and has returned to the same position which it occupied at the beginning of this immense period, which has been called the ‘great year.’ It is this retrograde evolution, in which the terrestrial axis describes round its own centre that revolution round a double conic surface, which is known as theprecession of the equinoxes. It was observed 2,000 years ago by Hipparchus; its cause was discovered by Newton; and its complete evolution explained by D’Alembert and Laplace.
“Now, we know that the consequence of the inclination of the terrestrial axis with the plane of the ecliptic is—
“1. That the seasons are inverse to the two hemispheres—that isto say, the northern hemisphere enjoys its spring and summer, while the southern hemisphere passes through autumn and winter.
“2. When the earth approaches nearest to the sun, our hemisphere has its autumn and winter; and the regions near the pole, receiving none of the solar rays, are plunged into darkness, approaching that of night, during six months of the year.
“3. When the earth is most distant from the sun, when much the greater half of the ecliptic intervenes between it and the focus of light and heat, the pole, being then turned towards this focus, constantly receives its rays, and the rest of the northern hemisphere enjoys its long days of spring and summer.
“Bearing in mind that, in going from the equinox of spring to the autumnal equinox of our hemisphere, the earth traverses a much longer curve than it does on its return; bearing in mind, also, the accelerated movement it experiences in its approach to the sun from the attraction, which increases in inverse proportion to the square of its distance, we arrive at the conclusion that our summer should be longer and our winter shorter than the summer and winter of our antipodes; and this isactuallythe case by about eight days.
“I sayactually, because, if we now look at the effects of the precession of the equinoxes, we shall see that in a time equal to half of thegrand year, whether it be 12,900 or 10,500 years, the conditions will be reversed; the terrestrial axis, and consequently the poles, will have accomplished the half of their bi-conical revolution round the centre of the earth. It will then be the northern hemisphere which will have the summers shorter and the winters longer, and the southern hemisphere exactly the reverse. In the year 1248 before the Christian era, according to M. Adhémar, the north pole attained its maximum summer duration. Since then—that is to say for the last 3,112 years—it has begun to decrease, and this will continue to the year 7388 of our era before it attains its maximum winter duration.
“But the reader may ask, fatigued perhaps by these abstract considerations, What is there here in common with the deluges?
“Thegrand yearis here divided, for each hemisphere, into two great seasons, which De Jouvencel calls the great summer and winter, which will each, according to M. Adhémar, be 10,500 years.
“During the whole of this period one of the poles has constantly had shorter winters and longer summers than the other. It follows that the pole which experiences the long winter undergoes a gradual and continuous cooling, in consequence of which the quantities of ice and snow, which melt during the summer, are more than compensatedby those which are again produced in the winter. The ice and snow go on accumulating from year to year, and finish at the end of the period by forming, at the coldest pole, a sort of crust or cap, vast, thick, and heavy enough to modify the spheroidal form of the earth. This modification, as a necessary consequence, produces a notable displacement of the centre of gravity, or—for it amounts to the same thing—of the centre of attraction, round which all the watery masses tend to restore it. The south pole, as we have seen, finished itsgreat winterin 1248b.c.The accumulated ice then added itself to the snow, and the snow to the ice, at the south pole, towards which the watery masses all tended until they covered nearly the whole of the southern hemisphere. But since that date of 1248, ourgreat winterhas been in progress. Our pole, in its turn, goes on getting cooler continually; ice is being heaped upon snow, and snow upon ice, and in 7,388 years the centre of gravity of the earth will return to its normal position, which is the geometrical centre of the spheroid. Following the immutable laws of central attraction, the southern waters accruing from the melted ice and snow of the south pole will return to invade and overwhelm once more the continents of the northern hemisphere, giving rise to new continents, in all probability, in the southern hemisphere.”
Such is a brief statement of the hypothesis which Adhémar has very ingeniously worked out. How far it explains the mysterious phenomena which we have under consideration we shall not attempt to say, our concern being with the effects. Does the evidence of upward and downward movements of the surface in Tertiary times explain the great change? For if the cooling which preceded and succeeded the two European deluges still remains an unsolved problem, its effects are perfectly appreciable. The intense cold which visited the northern and central parts of Europe resulted in the annihilation of organic life in those countries. All the watercourses, the rivers and streams, the seas and lakes, were frozen. As Agassiz says in his first work on “Glaciers”: “A vast mantle of ice and snow covered the plains, the valleys, and the seas. All the springs were dried up; the rivers ceased to flow. To the movements of a numerous and animated creation succeeded the silence of death.” Great numbers of animals perished from cold. The Elephant and Rhinoceros perished by thousands in the midst of their grazing grounds, which became transformed into fields of ice and snow. It is then that these two species disappeared, and seem to have been effaced from creation. Other animals were overwhelmed, without their race having been always entirely annihilated. Thesun, which lately lighted up the verdant plains, as it dawned upon these frozen steppes, was only saluted by the whistling of the north winds, and the horrible rending of the crevasses, which opened up on all sides under the heat of its rays, acting upon the immense glacier which formed the sepulchre of many animated beings.
How can we accept the idea that the plains, but yesterday smiling and fertile, were formerly covered, and that for a very long period, with an immense sheet of ice and snow? To satisfy the reader that the proof of this can be established on sufficient evidence, it is necessary to direct his attention to certain parts of Europe. It is essential to visit, at least in idea, a country whereglacial phenomenastill exist, and to prove that the phenomena, now confined to those countries, were spread, during geological times, over spaces infinitely vaster. We shall choose for our illustration, and as an example, the glaciers of the Alps. We shall show that the glaciers of Switzerland and Savoy have not always been restricted to their present limits; that they are, so to speak, only miniature resemblances of the gigantic glaciers of times past; and that they formerly extended over all the great plains which extend from the foot of the chain of the Alps.
To establish these proofs we must enter upon some consideration of existing glaciers, upon their mode of formation, and their peculiar phenomena.
The snow which, during the whole year, falls upon the mountains, does not melt, but maintains its solid state, when the elevation exceeds the height of 9,000 feet or thereabouts. Where the snow accumulates to a great thickness, in the valleys, or in the deep fissures in the ground, it hardens under the influence of the pressure resulting from the incumbent weight. But it always happens that a certain quantity of water, resulting from the momentary thawing of the superficial portions, traverses its substance, and this forms a crystalline mass of ice, with a granular structure, which the Swiss naturalists designatenévé. From the successive melting and freezing caused by the heat by day and the cold by night, and the infiltration of air and water into its interstices, thenévéis slowly transformed into a homogeneous azure mass of ice, full of an infinite number of little air-bubbles—this was what was formerly calledglace bulleuse(bubble-ice). Finally, these masses, becoming completely frozen, water replaces the bubbles of air. Then the transformation is complete; the ice is homogeneous, and presents those beautiful azure tints so much admired by the tourist who traverses the magnificent glaciers of Switzerland and Savoy.
Such is the origin of, and such is the mode in which the glaciersof the Alps are formed. An important property of glaciers remains to be pointed out. They have a general movement of translation in the direction of their slope, under the influence of which they make a certain yearly progress downward, according to the angle of the slope. The glacier of the Aar, for example, advances at the rate of about 250 feet each year.
Under the joint influence of the slope, the weight of the frozen mass, and the melting of the parts which touch the earth, the glacier thus always tends downwards; but from the effects of a more genial temperature, the lower extremity melting rapidly, has a tendency to recede. It is the difference between these two actions which constitutes the real progressive movement of the glacier.
The friction exercised by the glacier upon the bottom and sides of the valley, ought necessarily to leave its traces on the rocks with which it may happen to be in contact. Over all the places where a glacier has passed, in fact, we remark that the rocks are polished, levelled, rounded, and, as it is termed,moutonnées. These rocks present, besides, striations or scratches, running in the direction of the motion of the glacier, which have been produced by hard and angular fragments of stones imbedded in the ice, and which leave their marks on the hardest rocks under the irresistible pressure of the heavy-descending mass of ice. In a work of great merit, which we have before quoted, M. Charles Martins explains the physical mechanism by which granite rocks borne onwards in the progressive movements of a glacier, have scratched, scored, and rounded the softer rocks which the glacier has encountered in its descent. “The friction,” says M. Martins, “which the glacier exercises upon the bottom and upon the walls, is too considerable not to leave its traces upon the rocks with which it may be in contact; but its action varies according to the mineralogical nature of the rocks, and the configuration of the ground they cover. If we penetrate between the soil and the bottom of the glacier, taking advantage of the ice-caverns which sometimes open at its edge or extremity, we creep over a bed of pebbles and fine sand saturated with water. If we remove this bed, we soon perceive that the underlying rock is levelled, polished, ground down by friction, and covered with rectilinear striæ, resembling sometimes small grooves, more frequently perfectly straight scratches, as though they had been produced by means of a graver, or even a very fine needle. The mechanism by which these striæ have been produced is that which industry employs to polish stones and metals. We rub the metallic surface with a fine powder calledemery, until we give it a brilliancy which proceeds from the reflection of the light from aninfinity of minute striæ. The bed of pebbles and mud, interposed between the glacier and the subjacent rock, here represents the emery. The rock is the metallic surface, and the mass of the glacier which presses on and displaces the mud in its descent towards the plain, represents the hand of the polisher. These striæ always follow the direction of the glacier; but as it is sometimes subject to small lateral deviations, the striæ sometimes cross, forming very small angles with one another. If we examine the rocks by the side of a glacier, we find similar striæ engraved on them where they have been in contact with the frozen mass. I have often broken the ice where it thus pressed upon the rock, and have found under it polished surfaces, covered with striations. The pebbles and grains of sand which had engraved them were still encased in the ice, fixed like the diamond of the glazier at the end of the instrument with which he marks his glass.
“The sharpness and depth of the striæ or scratches depend on many circumstances: if the rock acted upon is calcareous, and the emery is represented by pebbles and sand derived from harder rocks, such as gneiss, granite, or protogine, the scratches are very marked. This we can verify at the foot of the glaciers of Rosenlaui, and of the Grindenwald in the Canton of Berne. On the contrary, if the rock is gneissic, granitic, or serpentinous, that is to say, very hard, the scratches will be less deep and less marked, as may be seen in the glaciers of the Aar, of Zermatt, and Chamounix. The polish will be the same in both cases, and it is often as perfect as in marble polished for architectural purposes.
“The scratches engraved upon the rocks which confine these glaciers are generally horizontal or parallel to the surface. Sometimes, owing to the contractions of the valley, these striæ are nearly vertical. This, however, need not surprise us. Forced onwards by the superincumbent weight, the glacier squeezes itself through the narrow part, its bulk expanding upwards, in which case the flanks of the mountain which barred its passage are marked vertically. This is admirably seen near the Châlets of Stieregg, a narrow defile which the lower glacier of the Grindenwald has to clear before it discharges itself into the valley of the same name. Upon the right bank of the glacier the scratches are inclined at an angle of 45° to the horizon. Upon the left bank the glacier rises sometimes quite up to the neighbouring forest, carrying with it great clods of earth charged with rhododendrons and clumps of alder, birches, and firs. The more tender or foliated rocks were broken up and demolished by the prodigious force of the glacier; the harder rocks offered moreresistance, but their surface is planed down, polished, and striated, testifying to the enormous pressure which they had to undergo. In the same manner the glacier of the Aar, at the foot of the promontory on which M. Agassiz’ tent was erected, is polished to a great height, and on the face, turned towards the upper part of the valley, I have observed scratches inclined 64°. The ice, erect against this escarpment, seemed to wish to scale it, but the granite rock held fast, and the glacier was compelled to pass round it slowly.
“In recapitulation, the considerable pressure of a glacier, joined to its movement of progression, acts at once upon the bottom and flanks of the valley which it traverses: it polishes all the rocks which may be too hard to be demolished by it, and frequently impresses upon them a peculiar and characteristic form. In destroying all the asperities and inequalities of these rocks, it levels their surfaces and rounds them on the sides pointing up the stream, whilst in the opposite direction, or down the stream, they sometimes preserve their abrupt, unequal, and rugged surface. We must comprehend, in short, that the force of the glacier acts principally on the side which is towards the circle whence it descends, in the same way that the piles of a bridge are more damaged up-stream, than down, by the icebergs which the river brings down during the winter. Seen from a distance, a group of rocks thus rounded and polished reminds us of the appearance of a flock of sheep: hence the nameroches moutonnéesgiven them by the Swiss naturalists.”
Another phenomenon which plays an important part in existing glaciers, and in those, also, which formerly covered Switzerland, is found in the fragments of rock, often of enormous size, which have been transported and deposited during their movement of progression.
The peaks of the Alps are exposed to continual degradations. Formed of granitic rocks—rocks eminently alterable under the action of air and water, they become disintegrated and often fall in fragments more or less voluminous. “The masses of snow,” continues Martins, “which hang upon the Alps during winter, the rain which infiltrates between their beds during summer, the sudden action of torrents of water, and more slowly, but yet more powerfully, the chemical affinities, degrade, disintegrate, and decompose the hardest rocks. The débris thus produced falls from the summits into the circles occupied by the glaciers with a great crash, accompanied by frightful noises and great clouds of dust. Even in the middle of summer I have seen these avalanches of stone precipitated from the highest ridges of the Schreckhorn, forming upon the immaculate snow a long black train, consisting of enormous blocks and an immensenumber of smaller fragments. In the spring a rapid thawing of the winter snows often causes accidental torrents of extreme violence. If the melting is slow, water insinuates itself into the smallest fissures of the rocks, freezes there, and rends asunder the most refractory masses. The blocks detached from the mountains are sometimes of gigantic dimensions: we have found them sixty feet in length, and those measuring thirty feet each way are by no means rare in the Alps.”[106]
Thus, the action of aqueous infiltrations followed by frost, the chemical decomposition which granite undergoes under the influence of a moist atmosphere, degrade and disintegrate the rocks which constitute the mountains enclosing the glacier. Blocks, sometimes of very considerable dimensions, often fall at the foot of these mountains on to the surface of the glacier. Were it immovable the débris would accumulate at its base, and would form there a mass of ruins heaped up without order. But the slow progression, the continuous displacement of the glacier, lead, in the distribution of these blocks, to a certain kind of arrangement: the blocks falling upon its surface participate in its movement, and advance with it. But other downfalls take place daily, and the new débris following the first, the whole form a line along the outer edge of the glacier. These regular trains of rocks bear the name of “moraines.” When the rocks fall from two mountains, and on each edge of the glacier, and two parallel lines of débris are formed, they are calledlateral moraines. There are alsomedian moraines, which are formed when two glaciers are confluent, in such a manner that thelateral moraine, on the right of the one, trends towards the left-hand one of the other. Finally, those moraines arefrontal, orterminal, which repose, not upon the glacier, but at its point of termination in the valleys, and which are due to the accumulation of blocks fallen from the terminal escarpments of glaciers there arrested by some obstacle. InPlate XXXI.we have represented an actual Swiss glacier, in which are united the physical and geological peculiarities belonging to these enormous masses of frozen water: the moraines here arelateral, that is to say, formed of a double line of débris.