This process of dividing rocks into small divisions has been carried to an extreme length among Jurassic and Cretaceous sediments, but has to some extent been applied to all systems. The Cambrian system, divided into groups primarily on the lith ology of the sediments of that age in north Wales, has been sub-divided into some 12 zones, chiefly on the evidence afforded by the range of different genera or species of trilobites. The suc ceeding Ordovician and Silurian systems were shown by Charles Lapworth to be divisible on the evidence of graptolites, and it has since been found that these graptolite zones can be recognised not only in the region of south Scotland, where they were estab lished, but throughout western Europe, North America and even Australia. They are thus of world-wide significance.
In the course of his work, Lapworth emphasized the fact that the Ordovician and Silurian rocks of south Scotland existed in two different forms—sandstones, shales and limestones, containing trilobites, the shells of brachiopods and molluscs, and corals ; and very thin-bedded usually black shales often crowded with graptolites but usually yielding very few other fossils. He called these two rock types the "Shelly" and the "Graptolite" facies respectively. Owing to the occasional occurrence of grapto lites in the shelly facies, he was able to show that the difference between the faunas of the two types did not depend on differences of time, but could be completely accounted for by the assumption that the rocks of the shelly facies were laid down near the coast in shallow water, where trilobites and shells were living. The graptolite shales on the other hand were deposited in relatively deep water and the graptolites whose remains were found in them were pelagic animals floating freely in the sea, their skeletons after the death of the colony sinking to the bottom and becoming buried in the scanty sediments which were there accumulating. The conception that two faunas of identical age may differ even completely on account of such a difference of facies, is a most im portant one, which has always to be kept in mind in discussing the geological age of any rock. Modern work on the distribution of animals on the floor of the Kattegat, the North Sea, and the English Channel, has shown conclusively that in very short dis tances, in some cases within a few hundred yards, totally different forms may be lying buried in the sea bottom ; the character of the fauna being determined in part by the nature of the soil, whether gravel, sand or mud, in part by the depth of water. (See MARINE BIOLOGY.) The fact that graptolites are unattached forms living floating in the surface layers of the water, explains the occurrence of their skeletons in both the shelly and graptolitic facies, and also ac counts for their extraordinarily wide distribution, whilst the fact that they were undergoing a rapid evolution is the explanation
their value for recognising small periods of time. The
zones of the Palaeozoic, however, probably cover long periods of time in comparison with those which are recognised in Jurassic rocks. Throughout the Mesozoic, zones have been established on the basis of the ammonites. These animals are often enormously abundant as individuals and were undergoing very rapid evolution from their first appearance at the base of the Permian to their disappearance at the top of the Cretaceous. Many of them are found in sediments of all types, individuals of the same species may be buried in sandstones, shales or limestones, a fact which suggests that like the graptolites, most of them at least did not dwell on the sea-bottom but swam freely in mid-waters or near the surface. These habits would explain the wide geographical range which some of them are known to have covered. For ex ample, the same or closely similar species of Mortoniceras occur in Texas, India, South Africa and Europe.
Ammonite zones may be and have been established in all marine sediments up to the top of the Cretaceous, but when we pass to Tertiary formations we enter into conditions in which no group of marine animals can be used for a world-wide division into zones. The difficulty of subdividing the Tertiary period in all probability is to be accounted for by the absence of any group of highly developed pelagic forms undergoing a rapid evolution and capable of ready preservation as fossils. A survey of the present fauna of surface waters, especially of those not entirely coastal, suggests that the most valuable groups for zonal purposes in the Tertiary would be the whales, whose size and rarity renders them unavailable; and the copepods which are, except in very special cases, incapable of preservation. Thus the sub division of the Tertiary period, so far as rocks of marine origin are concerned, necessarily rests on bottom-living forms from shallow seas, such as the Foraminif era, Gastropoda, Lamelli branchia and Echinoidea. It is perfectly clear from the present day distribution that species belonging to the last three groups have in general a restricted geographical distribution, and that they are very sensitive to changes of facies. The fundamental nature of the shortcomings which such materials possess when used as time-markers is vividly illustrated by the uncertainty which still exists as to the ages of the Tertiary deposit of Australia and South America. Indeed the Foraminifera, unsatisfactory as they theoretically should be, seem on the whole the most suitable fossils for determining the correlation of Tertiary rocks, at any rate over such a part of the world's surface as experienced tropical or warm conditions.