Heretofore the geological history of the earth has been studied only in the record of stratified rocks and their contained fossils. But in every place there have been land-periods in which, of course, erosion took the place of sedimentation. This kind of record is very imperfect, because there are no fossils. Until recently no account was taken of these erosion-periods except as breaks of indefinite length in the record--as lost intervals. But now, and mainly through the work of American geologists, interpretation of these erosion-periods has fairly commenced, and so important has this new departure in the study of geology seemed to some that it has been hailed as a new era in geology, connecting it more closely with geography. Heretofore _former_ land periods were recognized by unconformities and the amount of time by the degree of change in the fossils, but now the amount of time is estimated in _existing_ land surfaces by topographic _forms_ alone. This idea was introduced into geology by Major J. W. Powell, and has been applied with success by William Morris Davis, W. J. McGee, and others. The principle is this: Land surface subject to erosion and standing still is finally cut down to gently sweeping curves, with low, rounded divides and broad, shallow troughs. Such a surface is called by Davis a Peneplain. Such a peneplain is characteristic of old topography. If such a surface be again lifted to higher level, the rivers again dissect it by ravines, which are deep and narrow in proportion to the amount and rate of the uplift. If the land again remains steady, the sharply dissected surface is again slowly smoothed out to the gentle curves of a peneplain. If, on the contrary, the surface be depressed, the rivers fill up the channels with sediment which, on re-elevation, is again dissected. Thus the whole _ontogeny_ of land surfaces have been studied out, so that their age may be recognized at sight.