Fig. 12Fig. 12.—Comparative volume of the earth in the gaseous and solid state.
Fig. 12.—Comparative volume of the earth in the gaseous and solid state.
It has been calculated that the centre of the earth has a temperature of about 195,000° Cent., a degree of heat which surpasses all that the imagination can conceive. We can have no difficulty in admitting that, at a heat so excessive, all the substances which now enter into the composition of the globe would be reduced to the state of gas or vapour. Our planet, then, must have been originally an aggregation of aëriform fluids—a mass of matter entirely gaseous; and if we reflect that substances in their gaseous state occupy a volume eighteen hundred times larger than when solid, we shall have some conception of the enormous volume of this gaseous mass. It would be as large as the sun, which is fourteen hundred thousand times larger than the terrestrial sphere. InFig. 12we have attempted to give an idea of the vast difference of volume between the earth in its present solid state and in its primitive gaseous condition. One of the globes, A, represents the former, B the latter. It is simply a comparison of size, which is made the more strikingly apparent by means of these geometrical figures—onethe twentieth part of an inch in diameter, the other two inches and three quarters.
Plate VIVI.—The Earth circulating in space in the state of a gaseous star.
VI.—The Earth circulating in space in the state of a gaseous star.
At this excessive temperature the gaseous mass, which we have described, would shine in space as the sun does at the present day; and with the same brilliancy as that with which, to our eyes, the fixed stars and planets shine in the serenity of night, as represented on the opposite page (Plate VI.). Circulating round the sun in obedience to the laws of universal gravitation, this incandescent gaseous mass was necessarily regulated by the laws which govern other material substances. As it got cooler it gradually transferred part of its warmth to the glacial regions of the inter-planetary spaces, in the midst of which it traced the line of its flaming orbit. Consequent on its continual cooling (but at the end of a period of time of which itwould be impossible, even approximately, to fix the duration), the star, originally gaseous, would attain a liquid state. It would then be considerably diminished in volume.
The laws of mechanics teach us that liquid bodies, when in a state of rotation, assume a spherical form; it is one of the laws of their being, emanating from the Creator, and is due to the force of attraction. Thus the Earth takes the spheroidal form, belonging to it, in common with the greater number of the celestial bodies.
The Earth is subject to two distinct movements; namely, a movement of translation round the sun, and a movement of rotation on its own axis—the latter a uniform movement, which produces the regular alternations of days and nights. Mechanics have also established the fact, which is confirmed by experiment, that a fluid mass in motion produces (as the result of the variation of the centrifugal force on its different diameters), a swelling towards the equatorial diameter of the sphere, and a flattening at the poles or extremities of its axis. It is in consequence of this law, that the Earth, when it was in a liquid state, became swollen at the equator, and depressed at its two poles; and that it has passed from its primitive spherical form to the spheroidal—that is, has become flattened at each of its polar extremities, and has assumed its present shape of an oblate spheroid.
This bulging at the equator and flattening towards the poles afford the most direct proofs, that can be adduced, of the original liquid state of our planet. A solid and non-elastic sphere—a stone ball, for example—might turn for ages upon its axis, and its form would sustain no change; but a fluid ball, or one of a pasty consistence, would swell out towards the middle, and, in the same proportion, become flattened at the extremities of its axis. It was upon this principle, namely, by admitting the primitive fluidity of the globe, that Newton announcedà priorithe bulging of the globe at the equator and its flattening at the poles; and he even calculated the amount of this depression. The actual measurement, both of this expansion and flattening, by Maupertuis, Clairaut, Camus, and Lemonnier, in 1736, proved how exact the calculations of the great geometrican were. Those gentlemen, together with the Abbé Outhier, were sent into Lapland by the Academy of Sciences; the Swedish astronomer, Celsius, accompanied them, and furnished them with the best instruments for measuring and surveying. At the same time the Academy sent Bouguer and Condamine to the equatorial regions of South America. The measurements taken in both these regions established the existence of the equatorial expansion and the polar depression, as Newton had estimated it to be in his calculations.
It does not follow, as a consequence of the partial cooling down of the terrestrial mass, that all the gaseous substances composing it should pass into a liquid state; some of these might remain in the state of gas or vapour, and form round the terrestrial spheroid an outer envelope oratmosphere(from the Greek words ατμος,vapour, and σφαιρα,sphere). But we should form a very inexact idea of the atmosphere which surrounded the globe, at this remote period, if we compared it with that which surrounds it now. The extent of the gaseous matter which enveloped the primitive earth must have been immense; it doubtless extended to the moon. It included, in short, in the state of vapour, the enormous body of water which, as such, now constitutes our existing seas, added to all the other substances which preserve their gaseous state at the temperature then exhibited by the incandescent earth; and it is certainly no exaggeration to place this temperature at 2,000° Centigrade. The atmosphere would participate in this temperature; and acted on by such excessive heat, the pressure that it would exert on the Earth would be infinitely greater than that which it exercises at the present time. To the gases which form the component parts of the present atmospheric air—namely, nitrogen, oxygen, and carbonic acid—to enormous masses of watery vapour, must be added vast quantities of mineral substances, metallic or earthy, reduced to a gaseous state, and maintained in that state by the temperature of this gigantic furnace. The metals, the chlorides—metallic, alkaline, and earthy—sulphur, the sulphides, and even the silicates of alumina and lime; all, at this temperature, would exist in a vaporous form in the atmosphere surrounding the primitive globe.
It is to be inferred that, under these circumstances, the different substances composing this atmosphere would be ranged round the globe in the order of their respective densities; the first layer—that nearest to the surface of the globe—being formed of the heavier vapours, such as those of the metals, of iron, platinum, and copper, mixed doubtless with clouds of fine metallic dust produced by the partial condensation of their vapours. This first and heaviest zone, and the thickest also, would be quite opaque, although the surface of the earth was still at a red heat. Above it would come the more vaporisable substances, such as the metallic and alkaline chlorides, particularly the chloride of sodium or common salt, sulphur and phosphorus, with all the volatile combinations of these substances. The upper zone would contain matter still more easily converted into vapour, such as water (steam), together with others naturally gaseous, as oxygen, nitrogen, and carbonic acid. This order of superposition,however, would not always be preserved. In spite of their differences of density, these three atmospheric layers would often become mixed, producing formidable storms and violent ebullitions; frequently throwing down, rending, upheaving, and confounding these incandescent zones.
As to the globe itself, without being so much agitated as its hot and shifting atmosphere, it would be no less subject to perpetual tempests, occasioned by the thousand chemical actions which took place in its molten mass. On the other hand, the electricity resulting from these powerful chemical actions, operating on such a vast scale, would induce frightful electric detonations, thunder adding to the horror of this primitive scene, which no imagination, no human pencil could trace, and which constitutes that gloomy and disastrous chaos of which the legendary history of every ancient race has transmitted the tradition. In this manner would our globe circulate in space, carrying in its train the flaming streaks of its multiple atmosphere, unfitted, as yet, for living beings, and impenetrable to the rays of the sun, around which it described its vast orbit.
The temperature of the planetary regions is infinitely low; according to Laplace it cannot be estimated at less than 100° below zero. The glacial regions traversed in its course by the incandescent globe would necessarily cool it, at first superficially, when it would assume a pasty consistence. It must not be forgotten that the earth, on account of its liquid state, would be obedient in all its mass to the action of flux and reflux, which proceeds from the attraction of the sun and moon, but to which the sea alone is now subject. This action, to which all its liquid and movable particles were subject, would singularly accelerate the commencement of the solidification of the terrestrial mass. It would thus gradually assume that sort of consistence which iron attains, when it is first withdrawn from the furnace, in the process of puddling.
As the earth cooled, beds of concrete substances would necessarily be formed, which, floating at first in isolated masses on the surface of the semi-fluid matter, would in course of time come together, consolidate, and form continuous banks; just as we see with the ice of the present Polar Seas, which, when brought in contact by the agitation of the waves, coalesces and forms icebergs, more or less movable. By extending this phenomenon to the whole surface of the globe, the solidification of its entire surface would be produced. A solid, but still thin and fragile crust, would thus envelop the whole earth, enclosing entirely its still fluid interior.The entire consolidation would necessarily be a much slower process—one which, according to the received hypothesis, is very far from being completed at the present time; for it is estimated that the actual thickness of the earth’s crust does not exceed thirty miles, while the mean radius or distance from the centre of the terrestrial sphere, approaches 4,000 miles, the mean diameter being 7,912·409 miles; so that the portion of our planet, supposed to be solidified, represents only a very small fraction of its total mass.
We say thirty miles, for such is the ordinary estimated thickness of the earth’s crust, usually admitted by savants; and the following is the process by which this result has been obtained.
We know that the temperature of the earth increases one degree Centigrade for every hundred feet of descent. This resulthas been borne out by a great number of measurements, made in many of the mines of France, in the tin mines of Cornwall, in the mines of the Erzgeberge, of the Ural, of Scotland, and, above all, in the soundings effected in the Artesian wells of Grenelle and Passy, near Paris, of St. André de Iregny, and at a great number of other points.
The greatest depth to which miners have hitherto penetrated is about 973 yards, which has been reached in a boring executed in Monderf, in the Grand Duchy of Luxembourg. At Neusalzwerk, near Minden, in Prussia, another boring has been carried to the depth of 760 yards. In the coal-mines of Monkwearmouth the pits have been sunk 525 yards, and at Dukinfield 717 yards. The mean of the thermometic observations made, at all these points, leads to the conclusion that the temperature increases about one degree Fahrenheit for every sixty feet (English) of descent after the first hundred.
In admitting that this law of temperature exists for all depths of the earth’s crust, we arrive at the conclusion that, at a depth of from twenty-five to thirty-five miles—which is only about five times the height of the highest mountains—the most refractory matter would be in a state of fusion. According to M. Mitscherlich, the flame of hydrogen, burning in free air, acquires a temperature of 1,560° Centigrade. In this flame platinum would be in a state of fusion. Granite melts at a lower temperature than soft iron, that is at about 1,300°; while silver melts at 1,023°. In imagining an increase of temperature equal to one degree for every hundred feet of descent, the temperature at twenty-five miles will be 1,420° C. or 2,925° F.; thirty miles below the surface there will be a probable temperature of 1,584° C. or 3,630° F.; it follows, if these arguments be admitted, and the calculation correct, that the thickness of the solid crust of the globe does not much exceed thirty miles.
This result, which gives to the terrestrial crust a thickness equal to1⁄190of the earth’s diameter, has nothing, it is true, of absolute certainty.
Prof. W. Hopkins, F.R.S., an eminent mathematician, has much insisted upon the fact, that the conductibility of granite rocks, for heat, is much greater than that of sedimentary rocks; and he argues that in the lower stratum of the earth the temperature increases much more slowly than it does nearer the surface. This consideration has led Mr. Hopkins to estimate the probable thickness of the earth’s solid crust at a minimum of 200 miles.
In support of this estimate Mr. Hopkins puts forward another argument, based upon the precession of the equinoxes. We know that the terrestrial axis, instead of always preserving the same directionin space, revolves in a cone round the pole of the ecliptic. Our globe, it is calculated, will accomplish its revolution in about 25,000 years. In about this period it will return to its original position. This balancing, which has been compared to that of a top when about to cease spinning, produces the movement known as theprecession of the equinoxes. It is due to the attraction which the sun and moon exercise upon the swelling equatorial of the globe. This attraction would act very differently upon a globe entirely solid, and upon one with a liquid interior, covered by a comparatively thin crust. Mr. Hopkins subjected this curious problem to mathematical analysis, and he calculated that the precession of the equinoxes, observed by astronomers, could only be explained by admitting that the solid shell of the earth could not be less than from about 800 to 1,000 miles in thickness.
In his researches on therigidity of the earth, Sir William Thomson finds that the phenomena of precession and nutation require that the earth, if not solid to the core, must be nearly so; and that no continuous liquid vesicle at all approaching 6,000 miles in diameter can possibly exist in the earth’s interior, without rendering the phenomena in question very sensibly different from what they are.
The calculations of Mr. Hennessey are in direct opposition to those of Sir William Thomson, and show that the earth’s crust cannot be less than eighteen miles, or more than 600 miles in thickness.
Fig. 13Fig. 13.—Relative volumes of the solid crust and liquid mass of the globe.
Fig. 13.—Relative volumes of the solid crust and liquid mass of the globe.
Admitting, for the present, that the terrestrial crust is only thirty miles in thickness, we can express in a familiar, but very intelligible fashion, the actual relation between the dimensions of the liquid nucleus and the solid crust of the earth. If we imagine the earth to be an orange, a tolerably thick sheet of paper applied to its surface will then represent, approximately, the thickness of the solid crust which now envelopes the globe.Fig. 13will enable us to appreciate this fact still more correctly. The terrestrial sphere having a mean diameter of 7,912 miles, or a mean radius of 3,956 miles, and a solid crust about thirty miles thick, which is1⁄260of the diameter, or1⁄130of the radius, the engraving may be presumed to represent these proportions with sufficient accuracy.
To determine, even approximately, the time such a vast body would take in cooling, so as to permit of the formation of a solid crust, or to fix the duration of the transformations which we are describing, would be an impossible task.
Fig. 14Fig. 14.—Formation of primitive granitic mountains.
Fig. 14.—Formation of primitive granitic mountains.
The first terrestrial crust formed, as indicated, would be incapable of resisting the waves of the ocean of internal fire, which would be depressed and raised up at its daily flux and reflux in obedienceto the attraction of the sun and moon. Who can trace, even in imagination, the fearful rendings, the gigantic inundations, which would result from these movements! Who would dare to paint the sublime horrors of these first mysterious convulsions of the globe! Amid torrents of molten matter, mixed with gases, upheaving and piercing the scarcely consolidated crust, large crevices would be opened, and through these gaping cracks waves of liquid granite would be ejected, and then left to cool and consolidate on the surface.Fig. 14represents the formation of a primitive granitic mountain, by the eruption of the internal granitic matter which forces its way to the surface through a fracture in the crust. In some of these mountains, Ben Nevis for example, three different stages of the eruption can be traced. “Ben Nevis, now the undoubted monarch of the Scottish mountains,” says Nicol, “shows well the diverse age and relations of igneous rocks. The Great Moor from Inverlochy and Fort William to the foot of the hill is gneiss. Breaking through, and partly resting on the gneiss is granite, forming the lower two-thirds of the mountainup to the small tarn on the shoulder of the hill. Higher still is the huge prism of porphyry, rising steep and rugged all around.” In this manner would the first mountains be formed. In this way, also, might some metallic veins be ejected through the smaller openings, true injections of eruptive matter produced from the interior of the globe, traversing the primitive rocks and constituting the precious depository of metals, such as copper, zinc, antimony, and lead.Fig. 15represents the internal structure of some of these metallic veins. In this case the fracture is only a fissure in the rock, which soon became filled with injected matter, often of different kinds, which in crystallising would completely fill the hollow of this cleft, or crack; but sometimes forming cavities or geodes as a result of the contraction of the mass.
Fig. 15Fig. 15.—Metallic veins.
Fig. 15.—Metallic veins.
But some eruptions of granitic and other substances, ejected from the interior, never reach the surface at all. In such cases the clefts and crevices—longitudinal or oblique—are filled, but the fissures in the crust do not themselves extend to the surface.Fig. 16represents an eruption of granite through a mass of sedimentary rock—the granite ejected from the centre fills all the clefts and fractures, but it has not been sufficiently powerful to force its way to the surface.
Fig. 16Fig. 16.—Eruption of granite.
Fig. 16.—Eruption of granite.
On the surface of the earth, then, which would be at first smooth and unbroken, there were formed, from the very beginning, swelling eminences, hollows, foldings, corrugations, and crevices, which would materially alter its original aspect; its arid and burning surface bristled with rugged protuberances, or was traversed by enormous fissures and cracks. Nevertheless, as the globe continued to cool, a time arrived when its temperature became insufficient to maintain, in a state of vapour, the vast masses of water which floated in the atmosphere. These vapours would pass into the liquid state, and then the first rain fell upon the earth. Let us here remark that these were veritable rains of boiling water; for in consequence of the very considerable pressure of the atmosphere, water would be condensed and become liquid at a temperature much above 100° Centigrade (212° Fahr.)
Plate VIIVII.—Condensation and rainfall on the primitive globe.
VII.—Condensation and rainfall on the primitive globe.
The first drop of water, which fell upon the still heated terrestrial sphere, marked a new period in its evolution—a period the mechanical and chemical effects of which it is important to analyse. The contact of the condensed water with the consolidated surface of the globe opens up a series of modifications of which science may undertake the examination with a degree of confidence, or at least with more positive elements of appreciation than any we possess for the period of chaos; some of the features of which we have attempted to represent, leaving of necessity much to the imagination, and for the reader to interpret after his own fashion.
The first water which fell, in the liquid state, upon the slightly cooled surface of the earth would be rapidly converted into steam by the elevation of its temperature. Thus, rendered much lighter than the surrounding atmosphere, these vapours would rise to the utmost limits of the atmosphere, where they would become condensed afresh, in consequence of their radiation towards the glacial regions of space; condensing again, they would re-descend to the earth in a liquid state, to re-ascend as vapour and fall in a state of condensation. But all these changes, in the physical condition of the water, could only be maintained by withdrawing a very considerable amount of heat from the surface of the globe, whose cooling would be greatly hastened by these continual alternations of heat and cold; its heat would thus become gradually dissipated and lost in the regions of celestial space.
This phenomenon extending itself by degrees to the whole mass of watery vapour existing in the atmosphere, the waters covered the earth in increasing quantities; and as the conversion of all liquids into vapour is provocative of a notable disengagement of electricity, a vast quantity of electric fluid necessarily resulted from the conversion of such large masses of water into vapour. Bursts of thunder, and bright flashes of lightning were the necessary accompaniments of this extraordinary struggle of the elements—a state of things which M. Maurando has attempted to represent on the opposite page (Plate VII.).
How long did this struggle for supremacy between fire and water, with the incessant noise of thunder, continue? All that can be said in reply is, that a time came when water was triumphant. After having covered vast areas on the surface of the earth, it finally occupied and entirely covered the whole surface; for there is good reason to believe that at a certain epoch, at the commencement, so to speak, of its evolution; the earth was covered by water over its whole extent. The ocean was universal. From this moment our globeentered on a regular series of revolutions, interrupted only by the outbreaks of the internal fires which were concealed beneath its still imperfectly consolidated crust.
“At the early periods in which the materials of the ancient crystalline schists were accumulated, it cannot be doubted that the chemical processes which generated silicates were much more active than in more recent times. The heat of the earth’s crust was probably then far greater than at present, while a high temperature prevailed at comparatively small depths, and thermal waters abounded. A denser atmosphere, charged with carbonic acid gas, must also have contributed to maintain, at the earth’s surface, a greater degree of heat, though one not incompatible with the existence of organic life.
“These conditions must have favoured many chemical processes, which in later times have nearly ceased to operate. Hence we find that subsequently to the eozoic times, silicated rocks of clearly marked chemical origin are comparatively rare.”[32]
In order to comprehend the complex action, now mechanical, now chemical, which the waters, still in a heated state, exercised on the solid crust, let us consider what were the components of this crust. The rocks which formed its firststratum—the framework of the earth, the foundation upon which all others repose—may be presumed to have been a compound which, in varying proportions, forms granite and gneiss, and has latterly been designated by geologists Laurentian.
What is this gneiss, this granite, speaking of it with reference to its mineralogical character? It is a combination of silicates, with a base of alumina, potash, soda, and sometimes lime—quartz,felspar, andmicaform, by their simple aggregation,granite—it is thus a ternary combination, or composed of three minerals.
Quartz, the most abundant of all minerals, is silica more or less pure and often crystallised.Felsparis a crystalline or crystallised mineral, composed ofsilicateof alumina, potash, soda, or lime; potash-felspar is calledorthoclase, soda-felsparalbite, lime-felsparanorthite.Micais a silicate of alumina and potash, containing magnesia and oxide of iron; it takes its name from the Latinmicare, to shine or glitter.
Granite(from the Italiangrano, being granular in its structure) is, then, a compound rock, formed of felspar, quartz, and mica, and the three constituent minerals are more or less crystalline.Gneissis a schistose variety of granite, and composed of the same minerals;the only difference between the two rocks (whatever may be their difference of origin) being that the constituent minerals, instead of being confusedly aggregated, as in granite, assume a foliated texture in gneiss. This foliated structure leads sometimes to gneiss being calledstratified granite. “The term gneiss originated with the Freiberg miners, who from ancient times have used it to designate the rock in which their veins of silver-ore were found.”[33]
The felspar, which enters into the composition of granite, is a mineral that is easily decomposed by water, either cold or boiling, or by the water of springs rich in carbonic acid. The chemical action of carbonic acid and water, and the action (at once chemical and mechanical) of the hot water in the primitive seas, powerfully modified the granitic rocks which lay beneath them. The warm rains which fell upon the mountain-peaks and granitic pinnacles, the torrents of rain which fell upon the slopes or in the valleys, dissolved the several alkaline silicates which constitute felspar and mica, and swept them away to form elsewhere strata of clay and sand; thus were the first modifications in the primitive rocks produced by the united action of air and water, and thus were the first sedimentary rocks deposited from the oceanic waters.
The argillaceous deposits produced by this decomposition of the felspathic and micaceous rocks would participate in the still heated temperature of the globe—would be again subjected to long continued heat; and when they became cool again, they would assume, by a kind of semi-crystallisation, that parallel structure which is called foliation. All foliated rocks, then, are metamorphic, and the result of a metamorphic action to which sedimentary strata (and even some eruptive rocks) have been subjected subsequently to their deposition and consolidation, and which has produced a re-arrangement of their component mineral particles, and frequently, if not always, of their chemical elements also.
In this manner would the first beds of crystallineschist, such as mica-schist, be formed, probably out of sandy and clayey muds, or arenaceous and argillaceous shales.
At the end of this first phase of its existence, the terrestrial globe was, then, covered, over nearly its whole surface, with hot and muddy water, forming extensive but shallow seas. A few islands, raising their granitic peaks here and there, would form a sort of archipelago, surrounded by seas filled with earthy matter in suspension. During a long series of ages the solid crust of the globe went on increasing inthickness, as the process of solidification of the underlying liquid matter nearest to the surface proceeded. This state of tranquillity could not last long. The solid portion of the globe had not yet attained sufficient consistency to resist the pressure of the gases and boiling liquids which it covered and compressed with its elastic crust. The waves of this internal sea triumphed, more than once, over the feeble resistances which were opposed to it, making enormous dislocations and breaches in the ground—immense upheavals of the solid crust raising the beds of the seas far above their previous levels—and thus mountains arose out of the ocean, not now exclusively granitic, but composed, besides, of those schistose rocks which have been deposited under water, after long suspension in the muddy seas.
On the other hand the Earth, as it continued to cool, would also contract; and this process of contraction, as we have already explained, was another cause of dislocation at the surface, producing either considerable ruptures or simple fissures in the continuity of the crust. These fissures would be filled, at a subsequent period, by jets of the molten matter occupying the interior of the globe—byeruptive granite, that is to say—or by various mineral compounds; they also opened a passage to those torrents of heated water charged with mineral salts, with silica, the bicarbonates of lime and magnesia, which, mingling with the waters of the vast primitive ocean, were deposited at the bottom of the seas, thus helping to increase the mass of the mineral substances composing the solid portion of the globe.
These eruptions of granitic or metallic matter—these vast discharges of mineral waters through the fractured surface—would be of frequent occurrence during the primitive epoch we are contemplating. It should not, therefore, be a matter for surprise to find the more ancient rocks almost always fractured, reduced in dimensions by faults and contortions, and often traversed by veins containing metals or their oxides, such as the oxides of copper and tin; or their sulphides, such as those of lead, of antimony, or of iron—which are now the object of the miner’s art.
[32]“Address to the American Association for the Advancement of Science,” by Thomas Sterry Hunt, LL.D., p. 56. 1871.[33]Cotta’s “Rocks Classified and Described,” by P. H. Lawrence, p. 232.
[32]“Address to the American Association for the Advancement of Science,” by Thomas Sterry Hunt, LL.D., p. 56. 1871.
[33]Cotta’s “Rocks Classified and Described,” by P. H. Lawrence, p. 232.
After the terrible tempests of the primitive period—after these great disturbances of the mineral kingdom—Nature would seem to have gathered herself together, in sublime silence, in order to proceed to the grand mystery of the creation of living beings.
During the primitive epoch the temperature of the earth was too high to admit the appearance of life on its surface. The darkness of thickest night shrouded this cradle of the world; the atmosphere probably was so charged with vapours of various kinds, that the sun’s rays were powerless to pierce its opacity. Upon this heated surface, and in this perpetual night, organic life could not manifest itself. No plant, no animal, then, could exist upon the silent earth. In the seas of this epoch, therefore, only unfossiliferous strata were deposited.
Nevertheless, our planet continued to be subjected to a gradual refrigeration on the one hand, and, on the other, continuous rains were purifying its atmosphere. From this time, then, the sun’s rays, being less obscured, could reach its surface, and, under their beneficent influence, life was not slow in disclosing itself. “Without light,” said the illustrious Lavoisier, “Nature was without life; it was dead and inanimate. A benevolent God, in bestowing light, has spread on the surface of the earth organisation, sentiment, and thought.” We begin, accordingly, to see upon the earth—the temperature of which was nearly that of our equatorial zone—a few plants and a few animals make their appearance. These first generations of life will be replaced by others of a higher organisation, until at the last stage of the creation, man, endowed with the supreme attribute which we call intelligence, will appear upon the earth. “The wordprogress, which we think peculiar to humanity, and even to modern times,” said Albert Gaudry, in a lecture on the animals of the ancient world, delivered in 1863, “was pronounced by the Deity on the day when he created the first living organism.”
Did plants precede animals? We know not; but such would appear to have been the order of creation. It is certain that in thesediment of the oldest seas, and in the vestiges which remain to us of the earliest ages of organic life on the globe, that is to say, in the argillaceous schists, we find both plants and animals of advanced organisation. But, on the other hand, during the greater part of the primary epoch—especially during the Carboniferous age—the plants are particularly numerous, and terrestrial animals scarcely show themselves; this would lead us to the conclusion that plants preceded animals. It may be remarked, besides, that from their cellular nature, and their looser tissues composed of elements readily affected by the air, the first plants could be easily destroyed without leaving any material vestiges; from which it may be concluded, that, in those primitive times, an immense number of plants existed, no traces of which now remain to us.
We have stated that, during the earlier ages of our globe, the waters covered a great part of its surface; and it is in them that we find the first appearance of life. When the waters had become sufficiently cool to allow of the existence of organised beings, creation was developed, and advanced with great energy; for it manifested itself by the appearance of numerous and very different species of animals and plants.
Fig. 17Fig. 17.—Paradoxides Bohemicus—Bohemia.
Fig. 17.—Paradoxides Bohemicus—Bohemia.
One of the most ancient groups of organic remains are the Brachiopoda, a group of Mollusca, particularly typified by the genus Lingula, a species of which still exist in the present seas; the Trilobites (Fig. 17), a family of Crustaceans, especially characteristic of this period; then come Productas, Terebratulæ, and Orthoceratites—other genera of Mollusca. The Corals, which appeared at an early period, seem to have lived in all ages, and survive to the present day.
Contemporaneously with these animals, plants of inferior organisation have left their impressions upon the schists; these are Algæ (aquatic plants,Fig. 28). As the continents enlarged, plants of a higher type made their appearance—the Equisetaceæ, herbaceous Ferns, and other plants. These we shall have occasion to specify when noticing the periods which constitute the Primary Epoch, and which consists of the following periods: the Carboniferous, the Old Red Sandstone, and Devonian, the Silurian, and the Cambrian.
The researches of geologists have discovered but scanty traces of organic remains in the rocks which form the base of this system in England.Arenicolites, or worm-tracks and burrows, have been found in Shropshire, by Mr. Salter, to occur in countless numbers through a mile of thickness in the Longmynd rocks; and others were discovered by the late Dr. Kinahan in Wicklow. In Ireland, in the picturesque tract of Bray Head, on the south and east coasts of Dublin, we find, in slaty beds of the same age as the Longmynd rocks, a peculiar zoophyte, which has been named by Edward ForbesOldhamia, after its discoverer, Dr. Oldham, Superintendent of the Geological Survey of India. This fossil represents one of the earliest inhabitants of the ocean, which then covered the greater part of the British Isles. “In the hard, purplish, and schistose rocks of Bray Head,” says Dr. Kinahan,[34]“as well as other parts of Ireland which are recognised as Cambrian rocks, markings of a very peculiar character are found. They occur in masses, and are recognised as hydrozoic animal assemblages. They have regularity of form, abundant, but not universal, occurrence in beds, and permanence of character even when the beds are at a distance from each other, and dissimilar in chemical and physical character.” In the course of his investigations, Dr. Kinahan discovered at least four species of Oldhamia, which he has described and figured.
The Cambrian rocks consist of the Llanberis slates of Llanberis and Penrhyn in North Wales, which, with their associated sandy strata, attain a thickness of about 3,000 feet, and the Barmouth and Harlech Sandstones. In the Longmynd hills of Shropshire these last beds attain a thickness of 6,000 feet; and in some parts of Merionethshire they are of still greater thickness.
Neither in North Wales, nor in the Longmynd, do the Cambrian rocks afford any indications of life, except annelide-tracks and burrows. From this circumstance, together with general absence of Mollusca in these strata, and the sudden appearance of numerous shells and trilobites in the succeeding Lingula Flags, a change of conditions seems to have ensued at the close of the Cambrian period.
Believing that the red colour of rocks is frequently connected with their deposition in inland waters, Professor Ramsay conceives it to be possible, that the absence of marine mollusca in the Cambrian rocks may be due to the same cause that produced their absence inthe Old Red Sandstone, and that the presence of sun-cracks and rain-pittings in the Longmynd beds is a corroboration of this suggestion.[35]
The next period of the Primary Epoch is theSilurian, a system of rocks universal in extent, overspreading the whole earth more or less completely, and covering up the rocks of older age. The term “Silurian” was given by the illustrious Murchison to the epoch which now occupies our attention, because the system of rocks formed by the marine sediments, during the period in question, form large tracts of country in Shropshire and Wales, a region formerly peopled by theSilures, a Celtic race who fought gloriously against the Romans, under Caractacus or Caradoc, the British king of those tracts. The reader may find the nomenclature strange, as applied to the vast range of rocks which it represents in all parts of the Old and New World, but it indicates, with sufficient exactness, the particular region in our own country in which the system typically prevails—reasons which led to the term being adopted, even at a time when its vast geographical extent was not suspected.
Plate VIIIVIII.—Ideal Landscape of the Silurian Period.
VIII.—Ideal Landscape of the Silurian Period.
On this subject, and on the principles which have guided geologists in their classification of rocks, Professor Sedgwick remarks in one of his papers in theQuarterly Journal of the Geological Society: “In every country,” he says,[36]“which is not made out by reference to a pre-existing type, our first labour is that of determining the physical groups, and establishing their relations by natural sections. The labour next in order is the determination of the fossils found in the successive physical groups; and, as a matter of fact, the natural groups of fossils are generally found to be nearly co-ordinate with the physical groups—each successive group resulting from certain conditions which have modified the distribution of organic types. In the third place comes the collective arrangement of the groups into systems, or groups of a higher order. The establishment of the Silurian system is an admirable example of this whole process. The groups called Caradoc, Wenlock, Ludlow, &c., were physical groups determined by good natural sections. The successive groups of fossils were determined by the sections; and the sections, as the representatives of physical groups, were hardly at all modified by anyconsideration of the fossils, for these two distinct views of the natural history of such groups led to co-ordinate results. Then followed the collective view of the whole series, and the establishment of a nomenclature. Not only the whole series (considered as a distinct system), but every subordinate group was defined by a geographical name, referring us to a local type within the limits of Siluria; in this respect adopting the principle of grouping and nomenclature applied by W. Smith to our secondary rocks. At the same time, the older slate rocks of Wales (inferior to the system of Siluria), were calledCambrian, and soon afterwards the next great collective group of rocks (superior to the system of Siluria) was calledDevonian. In this way was established a perfect congruity of language. It was geographical in principle, and it represented the actual development of all our older rocks, which gave to it its true value and meaning.” The period, then, for the purposes of scientific description, may be divided into three sub-periods—the Upper and Lower Silurian, and the Cambrian.
Fig. 18Fig. 18.—Back of Asaphus caudatus (Dudley, Mus. Stokes), with the eyes well preserved. (Buckland.)
Fig. 18.—Back of Asaphus caudatus (Dudley, Mus. Stokes), with the eyes well preserved. (Buckland.)
Fig. 19Fig. 19.—a, Side view of the left eye of the above, magnified, (Buckland.)b, Magnified view of a portion of the eye of Calymene macrophthalmus. (Hœninghaus.)
Fig. 19.—a, Side view of the left eye of the above, magnified, (Buckland.)b, Magnified view of a portion of the eye of Calymene macrophthalmus. (Hœninghaus.)
The characteristics of the Silurian period, of which we give an ideal view opposite (Plate VIII.), are supposed to have been shallow seas of great extent, with barren submarine reefs and isolated rocks rising here and there out of the water, covered with Algæ, and frequented by various Mollusca and articulated animals. The earliest traces of vegetation belong to theThallogens, flowerless plants of the class Algæ (Fig. 28), without leaves or stems, which are found among the Lower Silurian rocks. To these succeed other plants, according to Dr. Hooker, belonging to the Lycopodiaceæ (Fig. 28), the seeds of which are found sparingly in the Upper Ludlow beds. Among animals,theOrthoceratitesled a predacious life in the Silurian seas. Their organisation indicates that they preyed upon other animals, pursuing them into the deepest abysses, and strangling them in the embrace of their long arms. TheTrilobites, a remarkable group of Crustacea, possessing simple and reticulated compound eyes, also highly characterise this period (Figs. 17to20); presenting at one period or other of their existence 1,677 species, 224 of which are met with in Great Britain and Ireland, as we are taught by the “Thesaurus Siluricus.”[37]Add to this a sun, struggling to penetrate the dense atmosphere of the primitive world, and yielding a dim and imperfect light to the first created beings as they left the hand of the Creator, organisms often rudimentary, but at other times sufficiently advanced to indicate a progress towards more perfect creations. Such is the picture which the artist has attempted to portray.
The elaborate and highly valuable “Thesaurus Siluricus” contains the names of 8,997 species of fossil remains, but it probably does not tell us of one-tenth part of the Silurian life still lying buried in rocks of that age in various parts of the world. A rich field is here offered to the geological explorer.[38]
The Silurian rocks have been estimated by Sir Roderick Murchison to occupy, altogether, an area of about 7,600 square miles in England and Wales, 18,420 square miles in Scotland, and nearly 7,000 square miles in Ireland. Thus, as regards the British Isles, the Silurian rocks rise to the surface over nearly 33,000 square miles.
The Silurian rocks have been traced from Cumberland to the Land’s End, at the southern extremity of England. They lie at the base of the southern Highlands of Scotland, from the North Channel to the North Sea, and they range along the entire western coast of that country. In a westerly direction they extended to the sea, where the mountains of Wales—the Alps of the great chain—would stand out in bold relief, some of them facing the sea, others in detached groups; some clothed with a stunted vegetation, others naked and desolate; all of them wild and picturesque. But an interest surpassing all others belongs to these mountains. They are amongst the most ancient sedimentary rocks which exist on our globe, a page of thebook in which is written the history of the antiquities of Great Britain—in fine, of the world.
In Shropshire and Wales three zones of Silurian life have been established. In rocks of three different agesGraptoliteshave left the trace of their existence. Another fossil characteristic of these ancient rocks is theLingula. This shell is horny or slightly calcareous, which has probably been one cause of its preservation. The family to which the Lingula belongs is so abundant in the rocks of the Welsh mountains, that Sir R. Murchison has used it to designate a geological era. These Lingula-flags mark the beginning of the first Silurian strata.
In the Lower Llandovery beds, which mark the close of the period, other fossils present themselves, thus greatly augmenting the forms of life in the Lower Silurian rocks. These are cœlenterata, articulata, and mollusca. They mark, however, only a very ephemeral passage over the globe, and soon disappear altogether.
The vertebrated animals are only represented by rare Fishes, and it is only on reaching the Upper Ludlow rocks, and specially in those beds which pass upward into the Old Red Sandstone, that the remains have been found of fishes—the most ancient beings of their class.