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that is to say, the external form is preserved, but the original organic elements have wholly disappeared, and have been replaced by foreign mineral substances—generally by silica or by carbonate of lime.
Fig. 1.—Labyrinthodon pachygnathus and footmarks.
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!
Fig. 2.—Impressions of rain-drops.