A great American geologist[Z] has made an attempt to remedy this by attaching definite values to such words as those above mentioned. In his system the greater divisions of the history were "Times:" thus the Eozoic was a time and the Palæozoic was a time. The larger divisions of the times are "Ages:" thus the Lower and Upper Silurian, the Devonian, and the Carboniferous are ages, which are equivalent in the main to what English geologists call Systems of Formations. Ages, again, may be divided into "Periods:" thus, in the Upper Silurian, the Ludlow of England, or Lower Helderberg of America, would constitute a period. These periods may again be divided into "Epochs," which are equivalent to what English geologists call Formations, a term referring not directly to the time elapsed, but to the work done in it. Now this mode of regarding geological time introduces many thoughts as to the nature of our chronology and matters relating to it. A "time" in geology is an extremely long time, and the Palæozoic was perhaps the longest of the whole. By the close of the Palæozoic nine-tenths of all the rocks we know in the earth's crust were formed. At least this is the case if we reckon mere thickness. For aught that we know, the Eozoic time may have accumulated as much rock as the Palæozoic; but leaving this out of the question, the rocks of the Palæozoic are vastly thicker then those of the Mesozoic and Cainozoic united. Thus the earth's history seems to have dragged slowly in its earlier stages, or to have become accelerated in its latter times. To place it in another point of view, life changes were greater relatively to merely physical changes in the later then in the earlier times. [Z] Dana. The same law seems to have obtained within the Palæozoic time itself. Its older periods, as the Cambrian and Lower Silurian, present immense thicknesses of rock with little changes in life. Its later periods, the Carboniferous and Permian, have greater life-revolution relatively to less thickness of deposits. This again was evidently related to the growing complexity and variety of geographical conditions, which went on increasing all the way up to the Permian, when they attained their maximum for the Palæozoic time.