CENOZOIC, se-no-zo'ik, ERA, the last of the great divisions commonly used in classify ing geological time, and therefore including the present. As it is the last, its records are much more complete than those of the Mesozoic, but while more complete they are, perhaps, more perplexing. Of the older eras— as, for instance, the Palaeozoic —much of the record left in the rocks has been obliterated, and thus only the traces of the greater changes in the distribution of land or water and of variations of climate have come down to us. Thus, while much is lost, the broader grouping of facts is easier. In the Cenozoic we have such a wealth of detail, such a great mass of evidence to sift and corre late, that geologists differ greatly as to how the record of the rocks shall be translated.
Generally speaking, the rocks of the Ceno zoic Era are less compacted than those of the Mesozoic, being very often beds of loose sand or clay. They usually lie horizontal, though sometimes upturned in a great mountain range. Any particular series is seldom of wide extent, and different series tell widely different stories of climatic conditions. Thus, probably no series in the Cenozoic in North America is comparable with the coal-bearing formations of the Creta ceous.
As to the climate of the Cenozoic, it was at first remarkably mild and even, Spitzbergen and Greenland having as mild a climate as that of Ohio to-day. Gradually the climate became colder, resulting in the great continental gla cier of the Ice Age. At a comparatively very recent date these glaciers receded, and the climate of the earth became substantially what it is to-day.
Cenozoic life is, on the whole, well differen tiated from Mesozoic, particularly by the great development of mammals and, probably well along in its last half (reckoning by actual time), by the advent of man. As mammals have de
veloped, so reptiles have declined, and to-day only snakes, crocodiles, lizards and turtles rep resent the class; the great ichthyosaurs, pleiso saurs, dinosaurs and pterosaurs were passing away by the end of the Cretaceous. In fact, as the Mesozoic was the era of reptiles, so is the Cenozoic the era of mammals. Among inverte brates many curious Mesozoic types have dis appeared, but of the genera in existence early in the Cenozoic most still exist. This also is true of Cenozoic plant life.
As has been noted, geologists differ in their divisions and subdivisions of Cenozoic time. American geologists generally make two great divisions, Tertiary and Quaternary.
The era is marked off from the Mesozoic, which precedes it, by the great disturbance which formed the Rocky Mountains. Through out, the continent of North America was largely out of water, with a few minor exceptions. The Atlantic and Gulf Coastal Plain underwent several successive submergences and emergen ces. The present site of the Coast Range was largely submerged till mid-Tertiary time, when a • pronounced, period of folding formed the Coast Ranges,- The Alps and Himalayas were probably also elevated at the same time. A period of. submergence occurred in the north east, at the close of the Glacial Period, in late Quaternary, which drowned Lake Ontario and Lake Champlain, separating New England as an island. (See also GEOLOGY; GLACIAL PERIOD; TERTIARY). Consult Dana, 'Manual of Geol ogy' ; Geikie, 'Text-book of Geology' ; Le Conte, 'Elements of Geology.>