In addition to the method of geologically dating the rocks by means of the fossils which they contain, there are other ways which may give a greater precision to the result. Climatic changes, when demonstrable, are of this character, for they may speedily and simultaneously affect vast areas of the earth’s surface or even the entire world. From time to time in the past, glacial conditions have prevailed over immense regions, several continents at once, it may be, as in one instance in which India, South Africa, Australia, South America were involved. The characteristic accumulations made by the glaciers in these widely separated regions must be contemporaneous in a sense that can rarely be predicated of the ordinary stratified rocks. Such climatic changes as the formation and disappearance of the ice-fields give a sharper and more definite standard of time comparisons than do the fossils alone, and yet the fossils are in turn needed to show which of several possible glacial periods are actually being compared. Again, great movements of the earth’s crust, which involve vast and widely separated regions and bring the sea in over great areas of land, or raise great areas into land, which had been submerged, may also yield more precise time-measurements, because occurring within shorter periods than do notable changes in the system of living things. Such changes in animals and plants may be compared to the almost imperceptible movement of the hour-hand of a clock, while the recorded climatic revolutions and crustal movements often supply the place of the minute-hand. It is obvious, however, that if the hour-hand be wanting, the minute-hand alone can be of very limited use. There have been a great many vast submergences and emergences of land in the history of the earth, and only the fossils can give us the assurance that we are comparing the same movement in distant continents, and not two similar movements separated by an enormous interval of time. It may thus fairly be admitted that it is possible to arrange the rocks which compose the accessible parts of the earth’s crust in chronological order and to correlate in one system the rocks of the various continents. The terms used for the more important divisions of geological time are, in descending order of magnitude, era, period, epoch, age or stage, and the general scheme of the eras and periods, which is in almost uniform use throughout the world, is given in the table, which is arranged so as to give the succession graphically, with the most ancient rocks at the bottom and the latest at the top. Cenozoic era { Quaternary period { Tertiary period { Cretaceous period Mesozoic era { Jurassic period { Triassic period