To the geologist fossils are of the greatest possible use, since they help him to determine the age of a particular set of strata, for certain kinds of fossils belong to certain rocks, and to them only.
But the classification of the stratified rocks has been carried farther than this. Practical geologists, working in the field, use fossils as their chief guide in working out the subdivisions of a group of rocks, for certain genera and species of old plants and animals are found to belong to certain small groups of strata. In this way a definite order of succession has been established once for all; and, except in the case of inverted strata already alluded to, this order is invariably found to hold good.
This great discovery of the order of succession of the British stratified rocks, established by their fossil contents, is due to William Smith, the father of English geology. After exploring the whole of England, he published in 1815 a geological map, the result of his extraordinary labours. Before then people had no idea of a definite and regular succession of rocks extending over the country, capable of being recognised to some extent by the nature of the rocks themselves,—whether sandstones, clays, or limestones, etc., but chiefly by their own fossils. They thought the different kinds of rocks were scattered promiscuously up and down the face of the country; but now we know that they do not show themselves in this haphazard way, but have definite relations to each other, like the many volumes of one large book.
By combining the two principles referred to above, geologists have arranged the great series of British stratified rocks into certain groups, each indicating a long period of time. First, they are roughly divided into three large groups, marking the three great eras into which geological time is divided. Secondly, these eras are further divided into certain periods. These periods are again divided into epochs, indicated by local divisions of their rocks. In this way we have something like a historical table. Omitting the small epochs of time, this table is as follows, in descending order:—
| Table of the British Stratified Rocks. | |||
|---|---|---|---|
| Era. | Period. | Prevailing Type. | |
| Cainozoic or Tertiary | { | Recent. Pleistocene, or Quaternary. Pliocene. Miocene. Eocene. | Mammals. |
| Mesozoic, or Secondary. | { | Cretaceous. Neocomian. Jurassic. Triassic. Permian. | Reptiles |
| Palæozoic, or Primary. | { | Carboniferous. Devonian, and Old Red Sandstone. Silurian. Cambrian. Archæan,[34] or Pew-Cambrian. | Fishes. Creatures without a backbone (invertebrates). |
The total thickness of all these rocks has been estimated at about one hundred thousand feet, or not far from twenty miles. These names have been given partly from the region in which the rocks occur, partly from the nature of the rocks themselves, and partly for other reasons. Thus the Old Red Sandstone is so called, because it generally, though not always, appears as a dark red sandstone. But the Silurian rocks, which we find in North Wales, receive their name from the Silures, an ancient Welsh tribe; the Cambrian rocks take theirs from Cambria, the old name for North Wales. The Cretaceous rocks are partly composed of chalk, for which the Latin word is creta; and so on. The terms "Palæozoic," "Mesozoic," and "Cainozoic" mean "ancient life," "middle life," and "recent or new life," thus indicating that as time went on the various types of life that flourished on the earth became less old-fashioned, and more like those prevailing at the present time. These used to be called "Primary," "Secondary," and "Tertiary;" but the terms were unfortunate, because the primary rocks, as then known, were not the first, or oldest. We have therefore included the Archæan rocks, since discovered, in this primary group. Only one fossil has been found in these rocks, and that is a doubtful one; hence they are sometimes called "Azoic," that is, "without life." The Mesozoic rocks are, as it were, the records of the "middle ages" in the world's history; while the Palæozoic take us back to a truly primeval time.
We have now learned how the geological age of any group of rocks may be determined. Thus, if a series of rocks of unknown age can be shown to rest on undoubtedly Silurian rocks in one place, and in another place to be overlaid or covered by undoubtedly Carboniferous rocks, they will probably belong to the Old Red Sandstone Period. If afterwards we find that they contain some of the well-known fossils of that period, the question of their age is settled at once. But we want more evidence than this. Suppose, now, we find somewhere on the flanks of a mountain-range a series of Permian and Triassic rocks, resting almost horizontally on disturbed and folded Carboniferous strata. Does not that at once prove that the upheaval took place before the Permian Period? Clearly it does, because the Permian rocks have evidently not been disturbed thereby. So now we can fix the date of our range of hills; namely, after the Carboniferous Period and before the Permian Period.
It is by such reasoning that the age of our Pennine range of hills, extending from the north of England into Derbyshire, has been fixed; for the Permian and Triassic strata lie undisturbed on the upheaved arch of Carboniferous rocks of which this chain is composed. Its structure is that of a broken and much denuded anticline, which stands up to form a line of hills only because the Carboniferous limestone is so much harder than the "coal measures," or coal-bearing rocks, on each side of it, that it has not been worn away so fast. In time, this great anticline will be entirely worn away like that of the Weald. It is called the Great Mountain Limestone, because it so often rises up to form high ground. The Mendip Hills in Somersetshire are of about the same date, and they too are largely composed of this great limestone formation.
Of course, a certain amount of up and down movement took place after the hills were upheaved, otherwise the Permian and Triassic rocks could not have been deposited on their sides; but these movements were slight and of a more general kind than those by which strata are thrown into folds.
The main upheaval, by which the rocks now forming the Highlands of Scotland were lifted up and contorted, took place after the Lower Silurian Period, and before that of the Old Red Sandstone; and there is clear evidence that even before the latter period they had not only been greatly altered, or "metamorphosed," by subterranean heat, but that they had suffered enormous denudation. And the work of carving out these mountains has gone on ever since; for even in Old Red Sandstone times they were probably not entirely covered by water. The Highland Mountains are therefore older than the Pennine range.