Fig. 8.—Unconformable superposition of Devonian conglomerate on Silurian slates, at St. Abb’s Head, Berwickshire.—After Lyell.
Putting together the facts thus obtained, we can frame a tabular arrangement of the earth’s strata, as in the table prefixed to this chapter; and when we add the further discovery, very early made by geologists, that the successive formations differ from each other in their fossil remains, we have the means of recognising any particular formation by its fossils, even when the stratigraphical evidence may be obscure or wanting. Thus our knowledge of Epochs of Life, and indeed of the whole geological history of the earth, is based on the superposition of beds in the earth’s crust, and on the diversity of fossil remains in the successive beds so superimposed on each other; and it is on these grounds that we are enabled to construct a Table of Geological Formations representing the whole series of beds as far as known, with the characteristic groups of fossils of each period. Here I might close these preliminary considerations, but there are a few accessory questions, important to our clear comprehension of the subject, which may profitably occupy our attention for a short time.
One of these relates to the absolute duration of the time represented by the geological history of the earth. Such estimates as our present knowledge enables us to form are very indefinite. Whether we seek for astronomical or geological data, we find great uncertainty. To such an extent is this the case, that current estimates of the time necessary to bring the earth from a state of primitive incandescence to its present condition have varied from fifteen millions of years to five hundred millions. Of the various modes proposed, perhaps the most satisfactory as well as instructive is that based on the rate of denudation of our present continents, as indicated by the amount of sediment carried down by great rivers. The Mississippi, draining a vast and varied area in temperate latitudes, is washing away the American land at the rate of one foot in 6,000 years. The Ganges, in a tropical climate and draining many mountain valleys, works at the rate of one foot in 2,358 years. The mean of these two great rivers would give one foot in 4,179 years, at which rate our continents would be levelled with the waters in about six millions of years. But the land has been in process of renewal as well as of waste in geological time; and a better measure will be afforded by the amount of beds actually deposited. The entire thickness of all the stratified rocks of Great Britain has been calculated by Ramsay at 72,000 feet. Now, if we suppose the waste in all geological time to have been on the average the same as at present, and that this material has been deposited to the thickness of 72,000 feet on a belt of sea margin 100 miles in width, we shall have about 86 millions of years as the time required.[1] This has the merit of approximating to Sir William Thomson’s calculation, based on the rate of cooling of the earth, that a minimum of 100 millions of years may represent the time since a solid crust first began to form. As it is more likely that the rate of denudation has on the average been greater in former geological periods than at present, we may perhaps estimate fifty or sixty millions of years as the time required for the accumulation of all our formations. Some geologists object to this as too little, but in this some of them are influenced by the exigencies of theories of evolution, and others appear to have no adequate conception of the vast lapse of time represented by such numbers, in its relation to the actual rates of denudation and deposition.
It should be mentioned here, however, that, on certain theories now somewhat generally accepted, respecting the nature and source of solar heat, the absolute duration of geological time would be much reduced below the estimate of Sir Wm. Thomson. Prof. Tait has based on such data an estimate of fifteen millions of years. Prof. Simon Newcomb says that “on the only hypothesis science will now allow us to make respecting the source of the solar heat” (the gravitation hypothesis of Helmholtz) “the earth was, twenty millions of years ago, enveloped in the fiery atmosphere of the sun.” Dr. Kirkwood has called attention to these results in connection with the planetary hypothesis of La Place, in the Proceedings of the American Philosophical Society.[2] Should such views prove to be well-founded, geological calculations as to the time required for the successive formations may have to be revised.
If now we attempt to divide this time among the formations known to us, according to their relative thicknesses, we have, according to an elaborate estimate of Professor Dana, the time ratios of 12, 3, and 1 for the Palæozoic, Mesozoic, and Cainozoic periods respectively. Taking the whole time since the beginning of the Cambrian as forty-eight millions of years, we should thus have for the Palæozoic thirty-six millions, for the Mesozoic nine, and for the Tertiary three. Another calculation, recently made by Professors Hull and Haughton, gives the following ratios:—
| Azoic | 34·3 per cent. |
| Palæozoic | 42·5 ” |
| Mesozoic and Cainozoic | 23·2 ” |
This calculation is, however, based on the absolute thickness of the several series as ascertained in Great Britain, without reference to the nature of the beds, as indicating different rates of accumulation. Under either estimate it will be seen that the Palæozoic time greatly exceeds the Mesozoic and Cainozoic together, and consequently that changes of life seem to have proceeded at an accelerated rate as time wore on.
Another inquiry of some importance relates to the manner of preservation of fossils, and the extent to which they constitute the material of rocks. This inquiry is doubly important, as it bears on the genuineness of fossil remains, and on the means we have of understanding their nature.
Some rocks are entirely made up of matter that once was alive, or formed part of living organisms. This is the case with some limestones, which consist of microscopic shells, or of larger shells, corals, and similar calcareous organisms, either entire or broken into fragments and cemented together with pasty or crystalline limestone filling their interstices. This may be seen in [Fig. 9], which represents a magnified slice of a Silurian limestone. Coal in like manner consists of carbonised vegetable matter, retaining more or less perfectly its organic structure, and sometimes even the external forms of its constituent parts. More frequently, fossils are dispersed more or less sparsely through the substance of beds composed of earthy matter; and they have usually been more or less affected by chemical changes, or by mechanical pressure, or are mineralised by different substances which have either filled their pores by infiltration or have more or less completely replaced their substance. Of course, as a rule, the softer and more putrescible organic matters have perished by decay, and it is only the harder and more resisting parts that remain. Even these have often yielded to the enormous pressure to which they have been subjected, and if at all porous, have been changed by the slow action of percolating water charged with various kinds of mineral matter in solution.