CHAPTER XXIV.

TERTIARY OR CAINOZOIC-EOCENE OLIGOCENE.

THE Cretaceous system closes the long succession of Secondary or Mesozoic formations. The rocks which come next in order are classed as Tertiary or Cainozoic. When these names were originally chosen, geologists generally believed not only that the divisions into which they grouped the stratified rocks of the earth's crust corresponded on the whole with well-defined periods of time, but that the abrupt transitions, so often traceable between systems of rocks, served to mark geological revolutions, in which old forms of life as well as old geographical conditions disappeared and gave place to new. One of the most notable of such breaks in the record was supposed to separate the Cretaceous system from all the younger rocks. This opinion arose from the study of the geology of Western Europe, and more especially of South-Eastern England and North-Western France. The top of the Chalk, partly worn down by denudation, was found to be abruptly succeeded by the pebble-beds, sands, and clays of the lower Tertiary groups. No species of fossils found in the Chalk were known to occur also in the younger strata. It was quite natural, therefore, that the hiatus at the top of the Cretaceous system should have been regarded as marking the occurrence of some great geological catastrophe and new creation, and, consequently, as one of the great divisional lines of the Geological Record.

More detailed investigation, however, has gradually overthrown this belief. In Northern France, Belgium, and Denmark various scattered deposits (Danian, p. 417) serve to bridge over the gap that was supposed to separate Mesozoic and Cainozoic formations. In the Alps, no satisfactory line could be found to separate undoubtedly Cretaceous strata from others as obviously Tertiary. And in various parts of the world, other testimony was gradually obtained to show that no general convulsion marked the end of the Secondary and beginning of the Tertiary periods, but that the changes on the earth's surface proceeded in the same orderly connection and sequence as during previous and subsequent geological ages. The break in the continuity of the deposits in Western Europe only means that in that part of the world the record of the intervening ages has not been preserved. Either strata containing the record were never deposited in the region in question, or, having been deposited, they have subsequently been removed.

Bearing in mind, then, that such geological terms are only used for convenience of classification and description, and that what is termed Mesozoic time glided insensibly into what is called Cainozoic, we have now to enter upon the consideration of that section of the earth's history comprised within the Tertiary or Cainozoic periods. The importance of this part of the geological chronicle may be inferred from the following facts. During Tertiary time the sea-bed was ridged up into land to such an extent as to give the continents nearly their existing area and contour. The crust of the earth was upturned into great mountain ranges, and notably into that long band of lofty ground stretching from the Pyrenees right through the heart of Europe and Asia to Japan. Some portions of the Tertiary sea-bed now form mountain peaks 16,000 feet or more above the sea. The generally warm climate of the globe, indicated by the world-wide diffusion of the same species of shells in Palæozoic and less conspicuously in Mesozoic time, now slowly passed into the modern phase of graduated temperatures, from great heat at the equator to extreme cold around the poles. At the beginning of the Tertiary or Cainozoic periods the climate was mild even far within the Arctic Circle, but at their close it became so cold that snow and ice spread far southward over Europe and North America.

The plants and animals of Tertiary time are strikingly modern in their general aspect. The vegetation consists, for the most part, of genera that are still familiar in the meadows, woodlands, and forests of the present day. The assemblage of animals, too, becomes increasingly like that of our own time as we follow the upward succession of strata in which the remains are preserved. In one strongly marked feature, however, does the Tertiary fauna stand contrasted alike with everything that preceded and followed it. If the Secondary periods could appropriately be grouped together under the name of the "Age of Reptiles," Tertiary time may not less fitly be called the "Age of Mammals." As the manifold reptilian types died out, the mammals, in ever-increasing complexity of organisation, took their place in the animal world. By the end of the Tertiary periods they had reached a variety of type and a magnitude of size altogether astonishing, and far surpassing what they now present. The great variety of pachyderms is an especially marked feature among them.

The rocks embraced under the terms Cainozoic or Tertiary have been classified according to a principle different from any followed with regard to the older formations. When they began to be sedulously studied, it was found that the percentage of recent species of shells became more numerous as the strata were followed from older to newer platforms. The French naturalist Deshayes determined the proportions of these species in the different Tertiary groups of strata, and the English geologist Lyell proposed a scheme of classification based on these ratios. His names, with modifications as to their application, have been generally adopted. They are compounds of the Greek καινος, recent, with affixes denoting the proportion of living species.

To the oldest Tertiary deposits, containing only about 3 per cent of living species of shells, the name Eocene (dawn of the recent) was given. The next series, containing a larger number of living species, has received the name of Oligocene (few recent). The third division in order is named Miocene, to indicate that the living species, though in still larger proportions, are yet a minority of the whole shells. The overlying series forming the uppermost of the Tertiary divisions is termed Pliocene (more recent), because the majority are now living species. The same system of nomenclature has been retained for the next overlying group, which forms the lowest member of the Post-tertiary or Quaternary series. This group is called Pleistocene (most recent), and all the species of shells in it are still living at the present time. It must not be supposed that the mere percentage of living or of extinct species of shells in a deposit always affords satisfactory evidence of geological age. Obviously, there may have been circumstances favourable or unfavourable to the existence of some shells on the sea-bottom which that deposit represents, or to the subsequent preservation of their remains. The system of classification by means of shell-percentages must be used with some latitude, and with due regard to other evidence of geological age.

EOCENE.

In Europe great geographical changes took place at the close of the Cretaceous period. The wide depression in which the Chalk had been deposited was gradually and irregularly elevated, and over its site a series of somewhat local deposits of clay, sand, marl, and limestone was laid down, partly in small basins of the sea-floor, and partly in estuaries, rivers, or lakes. In Southern Europe, however, the more open sea maintained its place, and over its floor were accumulated widespread and thick sheets of limestone which, from the crowded nummulites which they contain, are known as nummulitic limestone. These characteristic rocks extend all over the basin of the Mediterranean, stretching far into Africa and sweeping eastwards through the Alps, Carpathians, and Caucasus, across Asia to China and Japan. In North America, the rocks classed as Eocene present two contrasted types. Down the eastern and western borders of the Continent, from the coast of New Jersey into the Gulf of Mexico on the one side, and along the coast ranges of California and Oregon on the other, they are marine deposits, though occasionally presenting layers of lignite with terrestrial plants. Over the vast plateaux which support the Rocky Mountains, however,