But, for several reasons, sedimentary rocks formed in lakes are much more likely to contain the remains of land animals, than those that were formed in seas, and they are more likely to be in a complete state of preservation. Within the last century, five or six small lakes in Scotland, which had been artificially drained, yielded the remains of several hundred skeletons of stags, oxen, boars, horses, sheep, dogs, hares, foxes, and wolves. There are two ways in which these animals may have met with a watery grave. In the first place, they may have got mired on going into the water, or in trying to land on the other side, after swimming across. Any one who knows Scotch lakes will be familiar with the fact that their margins are often most treacherous ground for bathers. The writer has more than once found it necessary to be very cautious on wading into a lake while fishing, or in search of plants. Secondly, when such lakes are frozen over in winter, the ice is often very treacherous in consequence of numerous springs; and animals attempting to cross may be easily drowned. No remains of birds were discovered in these lakes, in spite of the fact that, until drained, they were largely frequented by water-fowl. But it must be remembered that birds are protected by their powers of flight from perishing in such ways as other animals frequently do. And, even should they die on the water, their bodies are not likely to be submerged; for, being light and feathery, they do not sink, but continue floating until the body rots away, or is devoured by some creature such as a hungry pike. For these reasons the remains of birds are unfortunately very rare in the stratified rocks; and hence our knowledge of the bird life of former ages is slight.
The Imperfection of the Record.
A very little consideration will serve to convince us that the record which Nature has kept in the stratified rocks is an incomplete one. There are many reasons why it must be so. It is not to be expected that these rocks should contain anything like a complete collection of the remains of the various tribes of plants and animals that from time to time have flourished in seas, lakes, and estuaries, or on islands and continents of the world. In endeavouring to trace the course of life on the globe at successive periods, we are continually met by want of evidence due to the “imperfection of the record”—to use Darwin’s phrase. The reasons are not far to seek. The preservation of organic remains, or even of impressions thereof, in sedimentary strata is, to some extent, a matter of chance. It is obvious that no wholly soft creature, such as a jelly-fish, can be preserved; although on some strata they have left impressions telling of their existence at a very early period.
A creature, to become fossilised, must possess some hard part, such as a shell, e.g. an oyster (fossil oysters abound in some strata); or a hard chitinous covering, like that of the shrimp, or the trilobites of Silurian times; or a skeleton, such as all the backboned (vertebrate) animals possess.
But even creatures that had skeletons have not by any means always been preserved. Bones, when left on the bottom of the sea, where no sediment, or very little, is forming, will decay, and so disappear altogether. As Darwin points out, we are in error in supposing that over the greater part of the ocean-bed of the present day sediment is deposited fast enough to seal up organic remains before they can decay. Over a large part of the ocean-bed such cannot be the case; and this conclusion has, of late years, been confirmed by the observations made during the fruitful voyage of H.M.S. Challenger in the Atlantic and Pacific Oceans.
Again, even in shallower parts of the old seas, where sand or mud was once deposited, fossilisation was somewhat accidental; for some materials, being porous, allow of the percolation of water, and in this way shells, bones, etc., have been dissolved and lost. Thus sandstone strata are always barren in fossils compared to shales and limestones, which are much less pervious. To take examples from our own country, the New Red Sandstone of the south-west of England, the midland counties, Cheshire, and other parts contains very few fossils indeed, while the clays and limestones of the succeeding Lias period abound in organic remains of all sorts. Even insects have left delicate impressions of their wings and bodies! while shells, corals, encrinites, fish-teeth, and bones of saurians are found in great numbers.
Again, it must be borne in mind that the series of stratified rocks known to geologists is not complete or unbroken. They have been well compared to the leaves of a book on history, of which whole chapters and many separate pages have been torn out. These gaps, or “breaks,” are due to what is called “denudation;” that is to say, a great many rocks, after having been slowly deposited in water, have been upraised to form dry land, and then, being subjected for ages to the destroying action of “rain and rivers,” or the waves of the sea, have been largely destroyed. Such rocks, in the language of geology, have been “denuded;” that is, stripped off, so that the underlying rocks are left bare.
But the process of rock-making does not go on continuously in any one area. Sedimentary strata have been formed in slowly sinking areas. But, if subsidence ceases, and the downward movement becomes an upward one, then the bed of the sea is converted into dry land, and the geological record is broken; for aqueous strata do not form on dry land. Blown sands and terrestrial lava-flows are exceptions; but such accumulations are very small and insignificant, and may therefore be neglected, especially as they contain no fossils.
In this way, as well as by the process of “denudation” already alluded to, breaks occur; and these breaks often represent long intervals of time. There are several such gaps in the British series of stratified rocks; and it is partly by means of these breaks, during which important geographical and other changes took place, that sedimentary rocks have been classified and arranged in groups representing geological periods. Thus, the Cainozoic, or Tertiary, rocks of the Thames' basin are separated by a long “break” from those of the preceding Cretaceous period. During that interval great changes in animal life took place, whereby, in the course of evolution, new types appeared on the scene. (See [Table of Strata, Appendix I].)
Another cause interfering with the record is to be found in those important internal changes that have taken place in stratified rocks—often over large areas—which may be ascribed to the influence of heat and pressure combined. This process of change, whereby soft deposits have been altered or “metamorphosed” into hard crystalline rocks, is known as “metamorphism.” Metamorphic rocks have lost not only their original structure and appearance, but also their included organic remains, or fossils. Thus, when a soft limestone has been converted by these means into crystalline statuary marble, any fossils it may once have contained have been destroyed. It is true that this applies more to older and lower deposits,—for the lowest are the oldest—but there can be no doubt that valuable records of the forms of life which peopled the world in former periods have been lost by this means.