“First, the same fossils may be traced over wide regions, if we examine strata in the direction of their planes, although by no means for indefinite distances.
“Secondly, while the same fossils prevail in a particular set of strata for hundreds of miles in a horizontal direction, we seldom meet with the same remains for many fathoms, and very rarely for several hundred yards, in a vertical line, or a line transverse to the strata. This fact has now been verified in almost all parts of the globe, and has led to a conviction, that at successive periods of the past, the same area of land and water has been inhabited by species of animals and plants even more distinct than those which now people the antipodes, or which now coexist in the Arctic, temperate, and tropical zones. It appears that, from the remotest periods, there has been ever a coming in of new organic forms, and an extinction of those which pre-existed on the earth, some species having endured for a longer, others for a shorter time; while none have ever reappeared after once dying out. The law which has governed the creation and extinction of species seems to be expressed in the verse of the poet—
‘Nature made him, and then broke the die;’
and this circumstance it is which confers on fossils their highest value as chronological tests, giving to each of them, in the eyes of the geologist, that authority which belongs to contemporary medals in history. The same cannot be said of each peculiar variety of rock; for some of these, as red-marl and red sandstone for example, may occur at once upon the top, bottom, and middle of the entire sedimentary series; exhibiting in each position so perfect an identity of mineral aspect, as to be undistinguishable. Such exact repetitions, however, of the same mixtures of sediment, have not often been produced, at distant periods, in precisely the same parts of the globe; and even where this has happened, we are seldom in any danger of confounding together the monuments of remote eras, when we have studied their imbedded fossils and relative position.”[[114]]
Let us now briefly explain the very simple but satisfactory basis on which this threefold division of the Tertiary rocks rests, and then proceed to a brief explanation of each. The reader will already have noted a statement on a preceding page, to which we shall be pardoned, if, a second time, we ask attention to it. In imparting elementary instruction on geology we have always found our classes more or less puzzled by the tertiary system, on account of its nomenclature and minute subdivisions, and we have learnt from experience the importance of presenting this statement over and over again. We have remarked that in the secondary rocks—that is, up to the end of the chalk system—there are no organic remains found precisely similar to any species existing at the present day; but when we come to the tertiary rocks, although we find many strangers, we find also a good many organic remains of the same kind and character as the shells, that are now found on our shores. In one part of the Tertiary the number of fossils that belongs to existing genera is many, in another more, in another more still; and upon this simple idea of positive, comparative, and superlative, the present division is based. Taking the percentage principle as a guide, Sir Charles Lyell and a distinguished French geologist, M. Deshayes, have ascertained that in the lowest beds of this system there were only 3½ per cent. of fossil shells similar to existing species, and this, for “the sake of clearness and brevity,” says Sir Charles, “was called the Eocene period, or the period of the dawn, the dawn of our modern era so far as its testaceous fauna are concerned.” Rising higher in the examination of these rocks, certain strata were found containing 18 per cent. of fossil shells, similar to shells found now; and to this was given the name of Miocene, the puzzling name already spoken of, because it means less recent, whereas it is in reality more recent, and is to be understood in relation to the series below, and not to the series above. A step higher up in this system revealed deposits of a coralline and craggy character, in which 41 per cent. of fossil shells like those of the present era were found; and to this the name of Pliocene, or more recent still, was given; and latterly, in Sicily chiefly, a series of strata has been discovered, referable to the Tertiary, in which 95 per cent. of recent species of shells have been found, and to this series the name of Post-Pliocene, or Pleistocene, has been given. Before our description of each of these divisions, let us add, that “the organic remains of the system constitute its most important and interesting feature. The fossils of earlier periods presented little analogy, often no resemblance, to existing plants and animals; here, however, the similitude is frequently so complete, that the naturalist can scarcely point out a distinction between them and living races. Geology thus unfolds a beautiful gradation of being, from the corals, molluscs, and simple crustacea of the grauwacke—the enamelled fishes, crinoidea, and cryptogamic plants of the lower secondary—the chambered shells, sauroid reptiles, and marsupial mammalia of the upper secondary—up to the true dicotyledonous trees, birds, and gigantic quadrupeds of the tertiary epoch. The student must not, however, suppose that the fossils of this era bring him up to the present point of organic nature, for thousands of species which then lived and flourished became in their turn extinct, and were succeeded by others long before man was placed on the earth as the head of animated existence. Of Plants, few marine species have been detected; but the fresh-water beds have yielded cycadeæ, coniferae, palms, willows, elms, and other species, exhibiting the true dicotyledonous structure. Nuts allied to those of the cocoa and other palms have been discovered in the London clay; and seeds of the fresh-water characeæ, or stoneworts, known by the name of gyrgonites (Gr., gyros, curved, and gonos, seed), are common in the same deposit. Of the Radiata, Articulata, and Mollusca, so many belong to existing genera, that this circumstance has suggested the classification of tertiary rocks according to the number of recent species which they contain.”—Chambers’ Outlines, p. 147.
Let us now begin the Eocene period. The most remarkable formations of this period are the London and Hampshire basins. Of the London basin we have already spoken in a previous chapter; a few additional remarks will be sufficient. The diagrams 4 and 5, p. 25, will explain these tertiary deposits better than any verbal explanation; and when it is remembered that this bed of clay is probably a thousand feet in thickness, we get a passing illustration of the folly of those puerile reports which a few years since were industriously circulated about a coming earthquake in London. Poor, uneducated people took up the alarm rather anxiously, never dreaming of what any tyro in science would have told them—that supposing there was a subterranean chimney on fire down below, there was a wet blanket under their feet composed of a thousand feet of sodden and solid clay, a blanket of the material they may see in the deep cuttings of the Great Northern Railway in and about London, that would most effectually have put out any fire, or checked the progress of any earthquake, just as a cannon-ball is stopped dead by a woolsack.
A run down the river Thames will take any one who has a day to spare to the isle of Sheppey, where he will be amply rewarded by seeing, on the north side of the island, an exposure of this formation in the cliff laid bare to the height of 200 feet, and which pleasure trip will be amply rewarded by the discovery in situ of the fossil tropical plants, &c., that once flourished in the neighbourhood of our cold and foggy London. “At the entrance of the Thames, the London clay extends on both sides of the river, and is admirably exhibited in the isle of Sheppey, which consists entirely of this stratum. The cliffs on the north side of the island are upwards of 200 feet high, and are cut down vertically by the action of the sea; they have long been celebrated for the remarkable abundance and variety of the organic remains obtained from them, amongst which, perhaps, the most interesting are the fruits, berries, and woody seed-vessels of several hundred species of plants. From the same locality there have also been obtained the remains of upwards of fifty species of fish, and a considerable number of crustaceans, and many other invertebrata; besides some remarkable bones which have been described by Professor Owen, and which indicate the former existence in this island of large serpents, and of such birds as prey upon small reptiles and mammalia. Many of these fossils, especially those of plants, are very difficult to preserve, owing to the great tendency of the iron pyrites, which enter largely into their composition, to effloresce and be destroyed by exposure to the atmosphere.”[[115]]
Passing from the London basin to the Hampshire basin or Barton beds, we shall first give a group of the shells found here; and we wish our readers could look at them as they lie before us in their condition of most exquisite preservation, so exquisite, that those who have seen them have involuntarily and frequently exclaimed, “But these can’t be fossils!” I know of no picture-painting of past history so touching, and yet so true, as these lovely specimens of the shells of the pre-Adamite condition of England in all their native simplicity. To those who see in them shells, and only shells, why, in the name of the prophet, give them figs, while we again remember Wordsworth’s hero,—
“A primrose on the river brim,
A yellow primrose was to him,