552 POPULAR SCIENCE MONTHLY. 



breaks of indefinite length in the record — as lost intervals. But 

 now, and mainlj through the work of American geologists, in- 

 terpretation of these erosion-periods has fairly commenced, and 

 so imjoortant 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 esti- 

 mated 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 chan- 

 nels 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. 



Thus, wliile heretofore the more recent movements of the 

 crust were supposed to be readable only on coast lines and by 

 means of the old sea strands, now we read with equal ease the 

 movements of the interior by means of the physiognomy of the 

 topography, and especially the structure of the river channels. 

 Moreover, while heretofore the history of the earth was supposed 

 to be recorded only in stratified rock and their contained fossils, 

 now we find that recent history is recorded and may be read also 

 in the general topography of the land surfaces. Geography is 

 studied no longer as mere description of earth forms, but also as 

 to the causes of these forms, no longer as to present forms, but 

 also as to the history of their becoming. Thus geogi'aphy, by its 

 alliance with geology, has become a truly scientific study, and as 

 such is now introduced into the colleges and universities. It is 

 this alliance with geology which has caused the dry bones of geo- 

 graphic facts to live. It is this which has created a soul under the 

 dry *' rihs of this death." This mode of study of the history of the 

 earth has just commenced. How much will come of it is yet to 

 be shown in the next century. 



