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CONFORMABILITY OF STRATA.
have been laid down at the expense of one of equal thickness, of the same extent of surface, we shall perceive that we can only know the lowest layers of the very thickest deposits that have occurred during the history of our globe. No wonder need be expressed that we cannot trace the changes between two forms of life occurring in closely superimposed strata if a thousand feet of rock has been worn away by the action of rain and sun between their now contiguous surfaces.
The fact that superimposed strata conform in their stratification to the beds beneath them, is in itself very insufficient evidence that denudation has not occurred; many of the Silurian strata are perfectly horizontal in Sweden, Russia, and Canada, although the mountain-chains above them consist of rocks deposited long after them * Gradual depression would be no more likely to alter their plane than the elevation to which they have evidently been subjected. Numerous changes of level must have occurred during the deposition of the conformable secondary strata of England, a fact evidenced by successive changes of climate, alternation of marine and freshwater beds, as well as by the presence of numerous layers of coal and trees fossilized in an upright position. The level Eocene strata have undoubtedly been upheaved, and
* Lyell, • Principles,' i. p. 315.
132 ANCIENT ROCKS SUBJECT TO DEGRADATION.
may be again depressed, without undergoing any deviation from the horizontal plane.
M. Elie de Beaumont * has traced more or less fully and conclusively the evidence that at least sixty, and perhaps more than a hundred, oscil. lations of level have occurred during the deposition of the strata of Europe, on geological grounds alone ; whilst palæontological research quite bears out the view that numerous changes between the relations of land and sea must have occurred, as it shows us numerous changes of climate took place, and alternations of deep sea and littoral forms are revealed; these are undoubted evidences of gradual upheaval and subsidence.
The change of a deposit from sand to calcareous rock or shale must have been accompanied by serious physical changes; and yet such beds con. tinually conform in their stratification to each other. Such changes must also have given rise to considerable migrations of the species living in each region, so that a totally new form would appear from migration. · We must further remember that the ancient Lias and Trias, with all the older rocks, are still undergoing destruction; so that in considering the probabilities of any of the more ancient periods having representative strata, it is by no means fair to . * Notice sur les Systèmes des Montagnes. Paris, 1852.
RARITY OF FOSSILIZATION.
compare them with the tertiary deposits : every stratum will evidently become more fragmentary in its record as time elapses, as every stratum is again liable to denudation and destruction, even after having remained covered for millions of years.
These considerations account in part for the fact that we never find a large series of transitional forms between two well-marked species or genera occurring in successive deposits, although we find the more ancient fossils less specialized than the more modern forms of life.
There is another reason why we should not find a vast number of graduated transitional forms; out of millions of existing individuals, only a few are preserved. If a dozen or a hundred are preserved in one place, it is because extremely favourable conditions existed there for preservation; so long as external conditions remained constant, fossilization would occasionally take place : if a modified form is deposited, it is only looked upon as another species. Much variation is not likely to occur without change of locality or condition; and both would materially interfere with the chance of preservation.
We have already seen that there is reason to believe that highly specialized groups, derived from a common ancestor, must have diverged contemporaneously and progressed pari passu ; and there.
fore we should not wonder at the discovery of the traces of birds and the bones of marsupial mammals in the trias, since reptiles, fish, and invertebrates were highly developed at that period.
There are, however, some special difficulties. It may fairly be asked, if the origin of the mammalia and birds were aquatic or marine, how is it no remains occur amongst those of the various reptiles which existed at a period when the mammalia must have been considerably specialized? Why do we not find mammalian bones amongst those of the Pterodactyles and Iguanodons of the Wealden But it may fairly be answered that it is extremely improbable that freshwater mammals existed in a region teaming with crocodiles ; if there were any, they probably lived high up in the rivers of that ancient continent; and the probability that their bones would have reached the large lakes and estuaries of the Wealden is small. The Galapagos archipelago has been called the Land of Reptiles; and the only existing mammal on the islands is a small mouse*. How erroneous would be the conclusion of a future geologist who concluded, on the evidence of strata deposited around the Galapagos, that the present era was characterized by reptiles and the absence of mammalia upon our globe!
* Lyell, “ Principles,' vol. i. p. 221.
EXTINCTION OF MESOZOIC REPTILIA.
· Indeed the character of the fauna of the Trias and Oolitic period itself was very different in other regions of our globe; for the bones of great fossil reptiles are unknown in these formations, except in Europe*
Perhaps in the Mesozoic era the arctic and antarctic regions abounded in mammalia and aquatic birds awaiting the extinction of the great saurian races to invade the temperate and tropical regions of the earth. It was probably the huge size of these reptilian inhabitants of the Wealden country which enabled them to hold the position of the dominant race; whilst their huge size was probably the very condition which brought about their extinction, just as the Mylodon, Megatherium, and Macrauchenia died out in America in more recent times. The reptilia of the Wealden may have disappeared with slightly changing conditions, or may have become extinct with the subsidence of the land when the waves of the great chalkocean invaded the estuaries and covered the forests of the Weald.
We may therefore safely infer that but few strata from amongst those deposited have lasted through the immeasurable æons which have intervened between the primary and recent periods. We know that these have only been very partially
* Louis Agassiz, ' Essay on Classification,' p. 154.