The history of the Cenozoic era brings us by gradual steps to the present order of things. Of no part of geological history have such full and diversified records been preserved as of the Cenozoic, and yet this very fulness is a source of difficulty and embarrassment when we attempt to arrange the various phenomena in their chronological order.
The sedimentary rocks of the Cenozoic era are, for the most part, quite loose and uncompacted; it is relatively rare to find hard rocks, such as so generally characterize the older formations. They are also most frequently undisturbed, retaining nearly their original horizontal positions, except when they have been upturned in the formation of great mountain chains. Another characteristic feature of Cenozoic strata is their locally restricted range; only in the oldest parts of the group do we find such widely extended formations as are common in the Palaeozoic and Mesozoic groups, and the later Cenozoic strata become more and more local in their character. This implies the restriction of the changes of level, the great transgressions and withdrawals of the sea no longer taking place as they had in the preceding eras. On the other hand, mountain making was effected on a very grand scale in the Cenozoic, and vuicanism was prevalent to an extent that seems never to have been reached before.
The climate of the era underwent some very remarkable and inexplicable changes. • At the beginning it resembled that of the Cretaceous in its generally mild and equable character, a luxuriant vegetation flourishing far within the Arctic Circle; but by very slow degrees and with many fluctuations, the climate grew colder, culminating in the Glacial Age, when much of the land in the northern hemisphere was covered with sheets of ice and snow and reduced to the condition of modern Greenland.
The life of the Cenozoic era is very clearly demarcated from that of the Mesozoic, though many modern characteristics began in the Cretaceous or even earlier. The peculiar Mesozoic Am-monoids, Belemnites, and many curious Bivalves disappeared almost entirely at the end of the Cretaceous, leaving only a few stragglers here and there to persist into the older Tertiary. Even more striking is the dwindling of the Reptiles; the Ichthyosaurs, Plesiosaurs, Mosasaurs, Dinosaurs, and Pterosaurs, which had given such a marked individuality to the Mesozoic fauna, have become totally extinct, leaving only Lizards and Snakes, Turtles and Crocodiles, and a few Choristodera to represent the class. But Cenozoic life is not distinguished from Mesozoic merely by negative characters; it has its positive features as well. The plants and invertebrate animals nearly all belong to genera which are still living, and the proportion of modern species steadily increases as we approximate the present time. The Fishes, Amphibia, and Reptiles differ but little from those of modern times, and the Birds take on the diversity and relative importance which characterize them now.
Above all, the Mammals undergo a wonderful expansion and take the place of the vanished reptiles, giving to Cenozoic time an altogether different character from all that went before it. The great geographical and climatic changes produced migrations of land animals and plants upon a grand scale, from continent to continent and from zone to zone, the result of which is the distribution of living beings over the earth's surface as we find it to-day.
There is some difference of usage regarding the subdivisions of the Cenozoic group, though the difference is principally with reference to the rank of those subdivisions. We shall follow the usual American practice of dividing the group into two systems, the Tertiary and Quaternary.