A CENTURY OF GEOLOGY, 288 



of American geoloo-ists. interpretation of those erosion periods has 

 fairly coninienced, 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 for- 

 mer 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 b}^ Maj. 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 topographv. 

 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 stead}^ the sharp! v 

 dissected surface is again slowly smoothed out to the gentle curves of a 

 peneplain. If, on the contrary, the surface be depressed, the rivers till 

 up the channels with sediment, which, on reelevation, is again dissected. 

 Thus the whole ontogeny of land surfaces have been studied out, so 

 that their age may be recognized at sight. 



Thus, while 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 Ave read with equal ease the movements of the interior 

 l)y means of the physiognonu" of the topography, and especiallv the 

 structure of the river channels. Moreover, while heretofore the his- 

 tory of the earth was supposed to be recorded only in stratified rock 

 and their contained fossils, now 'we tind that recent histor}' 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, 

 l)ut also as to the causes of these forms; no longer as to present forms, 

 but also as to the history of their becoming. Thus geography, 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 geographic facts to 

 live. It is this Avhich has created a soul under the dry ''ribs of this 

 death.'' This mode of study of the history of the earth has just com- 

 menced. How nuich will come of it is yet to be shown in the next 

 centur^^ 



In this connection it is interesting to trace the effect of environment 

 on geological reasonings in different countries. Heretofore, especially 

 in England, what we have called peneplains were usually attributed to 

 marine denudation— i. e., to cutting back of a coast line by constant 

 action of the waves, leaviup- l)ehind a level submarine plateau, which 



