Part 3

The success which attended Sir R. Murchison’s investigations soon attracted the attention of other geologists. Professor Sedgwick examined the older slaty strata, and succeeded in proving the position of the Cambrian rocks to be at the base of the Silurian. Still it was reserved for Sir William Logan, the Director of the Canadian Geological Survey, to establish the fact that immense masses of gneissic formation lay at the base of the Cambrian; and, by subsequent investigations, Sir Roderick Murchison satisfied himself that this formation was not confined to Canada, but was identical with the rocks termed by him Fundamental Gneiss, which exist in enormous masses on the west coast of Scotland, and which he proved to be the oldest stratified rocks in the British Isles. Subsequently he demonstrated the existence of these same Laurentian rocks in Bohemia and Bavaria, far beneath the Silurian rocks of Barrande.

While Murchison and Sedgwick were prosecuting their inquiries into the Silurian rocks, Hugh Miller and many others had their attention occupied with the Old Red Sandstone--the Devonian of Sedgwick and Murchison--which immediately overlies them. After a youth passed in wandering among the woods and rocks of his native Cromarty, the day came when Miller found himself twenty years of age, and, for the time, a workman in a quarry. A hard fate he thought it at the time, but to him it was the road to fame and success in life. The quarry in which he laboured was at the bottom of a bay formed by the mouth of a river opening to the south, a clear current of water on one side, as he vividly described it, and a thick wood on the other. In this silent spot, in the remote Highlands, a curious fossil fish of the Old Red Sandstone was revealed to him; its appearance struck him with astonishment; a fellow-workman named a spot where many such monuments of a former world were scattered about; he visited the place, and became a geologist and the historian of the “Old Red.” And what strange fantastic forms did it afterwards fall to his lot to describe! “The figures on a China vase or Egyptian obelisk,” he says, “differ less from the real representation of the objects than the fossil fishes of the ‘Old Red’ differ from the living forms which now swim in our seas.”

The Carboniferous Limestone, which underlies the coal, the Coal-measures themselves, the New Red Sandstone, the Lias, and the Chalk, have in their turn found their historians; but it would be foreign to our object to dwell further here on these particular branches of the subject.

Some few of the fossilised beings referred to resemble species still found living, but the greater part belong to species which have become altogether extinct. These fossil remains may constitute natural families, none of the genera of which have survived. Such is the Pterodactyle among Pterosaurian reptiles; the Ammonite among Mollusca; the Ichthyosaurus and the Plesiosaurus among the Enaliosaurian reptiles. At other times there are only extinct genera, belonging to families of which there are still some genera now living, as the genus Palæoniscus among fishes. Finally, in Tertiary deposits, we meet with some extinct species belonging to genera of our existing fauna: the Mammoth, for example, of the youngest Tertiary deposits, is an extinct species of the genus elephant.

Some fossils are terrestrial, like the gigantic Irish stag, Cervus Megaceros, the snail or Helix; fluviatile or lacustrine, like the Planorbis, the Lymnæa, the Physa, and the Unio; marine, or inhabiting the sea exclusively, as the Cowry (Cypræa), and the Oyster, (Ostrea).

Fossils are sometimes preserved in their natural state, or are but very slightly changed. Such is the state of some of the bones extracted from the more recent caves; such, also, is the condition of the insects found enclosed in the fossil resins in which they have been preserved from decomposition; and certain shells, found in recent and even in old formations, such as the Jurassic and Cretaceous strata--in some of which the shells retain their colours, as well as their brilliant pearly lustre or nacre. At Trouville, in Normandy, in the Kimeridge strata, magnificent Ammonites are found in the clay and marl, all brilliant with the colours of mother-of-pearl. In the Cretaceous beds at Machéroménil, some species of Ancyloceras and Hamites are found still covered with a nacre, displaying brilliant reflections of blue, green, and red, and retaining an admirable lustre. At Glos, near Liseaux, in the Coral Rag, not only the Ammonites, but the Trigoniæ and Aviculæ have preserved all their brilliant nacre. Sometimes these remains are much changed, the organic matter having entirely disappeared; it sometimes happens also, though rarely, that they become petrified, that is to say, the external form is preserved, but the original organic elements have wholly disappeared, and have been replaced by foreign mineral substances--generally by silica or by carbonate of lime.

Geology also enables us to draw very important conclusions from certain fossil remains whose true nature was long misunderstood, and which, under the name of coprolites, had given rise to much controversial discussion. Coprolites are the petrified excrements of extinct fossil animals. The study of these singular remains has thrown unexpected light on the habits and physiological organisation of some of the great antediluvian animals. Their examination has revealed the scales and teeth of fishes, thus enabling us to determine the kind of food in which the animals of the ancient world indulged: for example, the coprolites of the great marine reptile which bears the name of Ichthyosaurus contain the bones of other animals, together with the remains of the vertebræ, or of the phalanges (paddle-bones) of other Ichthyosauri; showing that this animal habitually fed on the flesh of its own species, as many fishes, especially the more voracious ones, do in our days.

The imprints left upon mud or sand, which time has hardened and transformed into sandstone, furnish to the geologist another series of valuable indications. The reptiles of the ancient world, the turtles in particular, have left upon the sands, which time has transformed into blocks of stone, impressions which evidently represent the exact moulds of the feet of those animals. These impressions have, sometimes, been sufficient for naturalists to determine to what species the animal belonged which thus left its impress on the wet ground. Some of these exhibit tracks to which we shall have occasion to refer; others present traces of the footprints of the great reptile known as the Labyrinthodon or Cheirotherium, whose footmarks slightly resemble the impression made by the human hand (Fig. 1). Another well-known impression, which has been left upon the sandstone of Corncockle Moor, in Dumfriesshire, is supposed to be the impress of the foot of some great fossil Turtle.

We may be permitted to offer a short remark on this subject. The historian and antiquary may traverse the battle-fields of the Greeks and Romans, and search in vain for traces of those conquerors, whose armies ravaged the world. Time, which has overthrown the monuments of their victories, has also effaced the marks of their footsteps; and of the many millions of men whose invasions have spread desolation throughout Europe, not even a trace of a footprint is left. Those reptiles, on the other hand, which crawled thousands of ages ago on the surface of our planet when it was still in its infancy, have impressed on the soil indelible proofs of their existence. Hannibal and his legions, the barbarians and their savage hordes, have passed over the land without leaving a material mark of their passage; while the poor turtle, which dragged itself along the silent shores of the primitive seas, has bequeathed to learned posterity the image and impression of a part of its body. These imprints may be perceived as distinctly on the rocks, as the traces left on moist sand or in newly-fallen snow by some animal walking under our own eyes. What grave reflections should be awakened within us at the sight of these blocks of hardened earth, which thus carry back our thoughts to the early ages of the world! and how insignificant seem the discoveries of the archæologist who throws himself into ecstacies before some piece of Greek or Etruscan pottery, when compared with these veritable antiquities of the earth!

The palæontologist (from παλαιος “ancient,” οντος “being,” λογος “discourse”), who occupies himself with the study of animated beings which have lived on the earth, takes careful account also of the sort of moulds left by organised bodies in the fine sediment which has enveloped them after death. Many organic beings have left no trace of their existence in Nature, except their impressions, which we find perfectly preserved in the sandstone and limestone, in marl or clay, and in the coal-measures; and these moulds are sufficient to tell us the kind to which the living animals belonged. We shall, no doubt, astonish our readers when we tell them that there are blocks of sandstone with distinct impressions of drops of rain which had fallen upon sea-shores of the ancient world. The impressions of these rain-drops, made upon the sands, were preserved by desiccation; and these same sands, being transformed by subsequent hardening into solid and coherent sandstones, their impressions have been thus preserved to the present day. Fig. 2 represents impressions of this kind upon the sandstone of Connecticut river in America, which have been reproduced from the block itself by photography. In a depression of the granitic rocks of Massachusetts and Connecticut, the red sandstone occupies an area of a hundred and fifty miles in length from north to south, and from five to ten miles in breadth. “On some shales of the finest texture,” says Sir Charles Lyell, “impressions of rain-drops may be seen, and casts of them in the argillaceous sandstones.” The same impressions occur in the recent red mud of the Bay of Fundy. In addition to these, the undulations left by the passage of the waters of the sea, over the sands of the primitive world, are preserved by the same physical agency. Traces of undulations of this kind have been found in the neighbourhood of Boulogne-sur-Mer, and elsewhere. Similar phenomena occur in a still more striking manner in some sandstone-quarries worked at Chalindrey (Haute-Marne). The strata there present traces of the same kind over a large area, and along with them impressions of the excrements of marine worms. One may almost imagine oneself to be standing on the sea-shore while the tide is ebbing.

CHEMICAL AND NEBULAR HYPOTHESES OF THE GLOBE.

Among the innumerable hypotheses which human ingenuity has framed to explain the phenomena which surround the globe, the two which have found most ready acceptance have been termed respectively the CHEMICAL, and the NEBULAR or mechanical hypothesis. By the first the solid crust is supposed to have contained abundance of potassium, sodium, calcium, magnesium, and other metallic elements. The percolating waters, coming in contact with these substances, produce combinations resulting in the conversion of the metals into their oxides--potash, soda, lime, and magnesia--all of which enter largely into the composition of volcanic rocks. The second hypothesis involves the idea of an original incandescent mass of vapour, succeeded by a great and still existing central fire.

This idea of a great central fire is a very ancient hypothesis: admitted by Descartes, developed by Leibnitz, and advocated by Buffon, it is supposed to account for many phenomena otherwise inexplicable; and it is confirmed by a crowd of facts, and adopted, or at least not opposed, by the leading authorities of the age. Dr. Buckland makes it the basis of his Bridgewater treatise. Herschel, Hind, Murchison, Lyell, Phillips, and other leading English astronomers and geologists give a cautious adhesion to the doctrine. The following are some of the principal arguments adduced in support of the hypothesis, for, in the nature of the proofs it admits of, it can be no more.

When we descend into the interior of a mine, it is found that the temperature rises in an appreciable manner, and that it increases with the depth below the surface.

The high temperature of the waters in Artesian wells when these are very deep, testifies to a great heat of the interior of the earth.

The thermal waters which issue from the earth--of which the temperature sometimes rises to 100° Centigrade and upwards--as, for instance, the Geysers of Iceland--furnish another proof in support of the hypothesis.

Modern volcanoes are said to be a visible demonstration of the existence of central heat. The heated gases, the liquid lava, the flames which escape from their craters, all tend to prove sufficiently that the interior of the globe has a temperature prodigiously elevated as compared with that at its surface.

The disengagement of gases and burning vapours through the accidental fissures in the crust, which accompany earthquakes, still further tends to establish the existence of a great heat in the interior of the globe.

We have already said that the temperature of the globe increases about one degree for every sixty or seventy feet of depth beneath its surface. The correctness of this observation has been verified in a great number of instances--indeed, to the greatest depth to which man has penetrated, and been able to make use of the thermometer. Now, as we know exactly the length of the radius of the terrestrial sphere, it has been calculated from this progression of temperature, supposing it to be regular and uniform, that the centre of the globe ought to have at the present time a mean temperature of 195,000° Centigrade. No matter could preserve its solid state at this excessive temperature; it follows, then, that the centre of the globe, and all parts near the centre, must be in a permanent state of fluidity.

The works of Werner, of Hutton, of Leopold von Buch, of Humboldt, of Cordier, W. Hopkins, Buckland, and some other English philosophers, have reduced this hypothesis to a theory, on which has been based, to a considerable extent, the whole science of modern geology; although, properly speaking, and in the popular acceptation of the term, that science only deals with the solid crust of the earth.

The nebular theory thus embraces the whole solar system, and, by analogy, the universe. It assumes that the SUN was originally a mass of incandescent matter, that vast body being brought into a state of evolution by the action of laws to which the Creator, in His divine wisdom, has subjected all matter. In consequence of its immense expansion and attenuation, the exterior zone of vapour, expanding beyond the sphere of attraction, is supposed to have been thrown off by centrifugal force. This zone of vapour, which may be supposed at one time to have resembled the rings of Saturn, would in time break up into several masses, and these masses coalescing into globes, would (by the greater power of attraction which they would assume as consolidated bodies) revolve round the sun, and, from mechanical considerations, would also revolve with a rotary motion on their own axes.

This doctrine is applied to all the planets, and assumes each to have been in a state of incandescent vapour, with a central incandescent nucleus. As the cooling went on, each of these bodies may be supposed to have thrown off similar masses of vapour, which, by the operation of the same laws, would assume the rotary state, and, as satellites, revolve round the parent planet. Such, in brief, was the grand conception of Laplace; and surely it detracts nothing from our notions of the omnipotence of the Creator that it initiates the creation step by step, and under the laws to which matter is subjected, rather than by the direct fiat of the Almighty. The hypothesis assumes that as the vaporous mass cooled by the radiation of heat into space, the particles of matter would approximate and solidify.

That the figure of the earth is such as a very large mass of matter in a state of fluidity would assume from a state of rotation, seems to be admitted, thus corroborating the speculations of Leibnitz, that the earth is to be looked on as a heated fluid globe, cooled, and still cooling at the surface, by radiation of its superfluous heat into space. Mr. W. Hopkins has put forth some strong but simple reasons in support of a different theory; although he does not attempt to solve the problem, but leaves the reader to form his own conclusions. As far as we have been able to follow his reasoning we gather from it that:--

See Phil. Transactions, 1839-40-42; also, Quarterly Journal of the Geological Society, vol. viii., p. 56.

If the earth were a fluid mass cooled by radiation, the cooled parts would, by the laws of circulating fluids, descend towards the centre, and be replaced on the surface by matter at a higher temperature.

The consolidation of such a mass would, therefore, be accompanied by a struggle for superiority between pressure and temperature, both of which would be at their maximum at the centre of the mass.

At the surface, it would be a question of rapidity of cooling, by radiation, as compared with the internal condition--for comparing which relations we are without data; but on the result of which depends whether such a body would most rapidly solidify at the surface by radiation, or at the centre by pressure.

The effect of the first would be solidification at the surface, followed by condensation at the centre through pressure. There would thus be two masses, a spherical fluid nucleus, and a spherical shell or envelope, with a large zone of semi-fluid, pasty matter between, continually changing its temperature as its outer or inner surface became converted to the solid state.