HISTORICAL GEOLOGY.
93. The fossiliferous strata, as they are generally termed, have been chronologically arranged in a series of formations, each of which is characterised by its own peculiar suites of fossils. Their relative age has been determined, as we have indicated above, by their fossils, and also by certain physical tests, the chief of these being superposition. It holds invariably true that a formation, A, found resting upon another series of strata, B, will always occur in precisely the same position, wherever these two deposits occur together. If B should appear in some place as resting upon A, we may be sure that the beds have been inverted during the contortion of the strata consequent upon subterranean action (see [fig. 11], page 34). Again, another useful test of the relative age of strata lies in the circumstance that one is often made up or contains fragments of the other. In this case, then, it is quite clear which is the more recent accumulation. These tests have now been applied to the strata in many parts of the world, and the result is that geologists have been able to arrive at a chronological arrangement or classification, and so to construct a table shewing the relative position which would be occupied by all the different formations, if these occurred together in one place. In the British Islands the long series of strata is well developed, but many of the formations are much more meagrely represented than their equivalents in other countries. But even when we attempt to fill up the blanks in our own series by dovetailing with them the strata of foreign countries, there yet remain numerous breaks in the succession, pointing to the fact that the stony record is a very fragmentary one at the best. No doubt there are many large tracts of the earth's surface which have not yet been investigated, and when these are known we may hope to have our knowledge greatly increased. But no one who reflects upon the mode of origin of the fossiliferous strata, and the wonderful mutations which the earth has undergone, can reasonably anticipate that a perfect and complete record of the geological history of our planet shall ever be compiled from the broken and fragmentary testimony of the rocks.
94. The following table gives the names of the different formations arranged in the order of their superposition, the youngest being at the top, and the oldest known at the bottom:
95. The Primary formations are so called because they are the oldest known to us: they are not necessarily the first-formed aqueous deposits. Dr Hutton said truly: There is no trace of a beginning, and no signs of an end. In the Primary or Palæozoic (ancient-life) formations are found the earliest traces of life. The forms as a rule depart very widely from those with which we are acquainted now. The Laurentian rocks have yielded only one fossil—a large foraminifer named Eozoon Canadense. The Cambrian formation contains but few fossils—crustaceans, molluscs, zoophytes, and worm-tracks. The Silurian strata are often abundantly fossiliferous. All the great classes of invertebrates are represented, and fish remains also occur. The Devonian and Old Red Sandstone are also characterised by the presence of an abundant fauna. In the Old Red Sandstone are numerous fish remains; it appears to have been an estuarine or lacustrine deposit; the Devonian, on the other hand, was marine, like the Silurian and Cambrian. The Carboniferous formation is the chief repository of coal in Britain. It consists of terrestrial, fresh or brackish water, and marine deposits. The fauna and flora of the Permian, which is partly a marine and partly a fresh-water formation, are allied, upon the whole, to those of the Carboniferous, but offer at the same time many contrasts.
96. The Secondary or Mesozoic (middle-life) formations contain assemblages of fossils which do not depart so widely from analogous living forms as those belonging to Palæozoic times. The Triassic strata yield abundance of rock-salt. In Britain they contain very few fossils, but these are more abundant in the Triassic deposits of foreign countries. The oldest known mammals first appear in this formation. The Jurassic formation is very highly fossiliferous. It is distinguished by the occurrence of numerous reptilian remains. Nearly all the beds of this formation are marine, but there are associated with these the remains of a forest or old land surface, and a considerable accumulation of estuarine or fresh-water deposits; impure coals also occur in this formation. The Cretaceous strata are almost wholly marine, and chiefly of deep-water origin. But some land-plants are found, chiefly ferns, conifers, and cycads. Near the base of the formation occurs a great river deposit (Weald clay) with numerous remains of reptiles.
97. Among the oldest strata of the Tertiary or Cainozoic (recent-life) division we meet with the dawn of the existing state of the testaceous fauna—the Eocene (ēos, dawn, and kainos, recent) containing three and a half per cent. of recent species among its shells. The proportion of recent species increases in the Miocene (meion, less, and kainos, recent), although the majority of the molluscs entombed in that formation belong to extinct species. In the Pliocene (pleion, more, and kainos, recent), however, the extinct species are in a minority.