7HH J. BAKRKJ.r. M.EASI;KEMENTS ok (GEOLOGIC TIME 



The liigJi rate of erosion of lowlands is indicated b}' the ligures for 

 (lenndation ol)tained by Dole and Stabler. A difference between these 

 aird the nionntain regions would become apparent in time if the diastro- 

 phism were to enter on a slowing pulsation or to cease altogether. With 

 geological rapidity the lowland valleys would widen out, the average slopes 

 become much reduced, and the rate of erosion fall to a fraction of its 

 former rate. Tn the mountains, on the other hand, erosion would con- 

 tinue witli undiminished rate for a time and would, only slowly diminish, 

 the mountains remaining mature long after the lowlands had become old. 



The conclusion tliat the Pliocene-Pleistocene movements are members 

 of a rpiickening series is evident for the Atlantic shores. Around the 

 Pacific the movements are on a more prodigious scale ; but there again, 

 especially in the Andes, is seen the evidence of great intrenchment of 

 the rivers flowing through profound gorges. Thousands of feet above is 

 an older mature to]iogra])hy that s]ieaks of a long time of rpiieter crust 

 and' lessened rate of erosion. 



The present rate of denudation of the lands may then be twice or thrice 

 the average for the Cenozoic and five or ten times the average for earlier 

 ]:)eriods. Nevertheless, many ancient sedimentary formations testify to 

 rivers as swift as those of today. The Old Eed sandstone consists of de- 

 posits analogous to those now foi^ning in Cordilleran or Eurasian inter- 

 montane basins. Such formations are, however, relatively local. They 

 aie, furthermore, recurrent, not continuous, in time. Tliey emphasize by 

 contrast the low rates and limited areas of denudation diaracteristic of 

 intervening periods. 



The association of glaciation Avithin periods of revolution indicates that 

 one of the ulterior causes of glacial climates lies in diastropldsm. The 

 individual advances and retreats of the continental ice can not. however, 

 be correlated with individual crust movements; nevertheless, it is sug- 

 gestive of the quickening diastrophism that the Grlacial e])()chs appear 

 to have been more quickly recurrent in the later Pleistocene. The relative 

 remoteness of the Glacial epochs is given by ( 'haml)erlin and Salisbury as 

 follows : 



From tne late Wisconsin to the present 1 



From the early Wisconsin to the present 2 to 2^2 



From the Towan to the present 8 to 5 



From the Illinoian to the present 7 to 9 



From the Kansan to the present 15 to 17 



From the suh-Af tonian to the present X ^''' 



IS Chambprlin and Salislniry : Geoloay. vol. ili. 1906, p. 414. 



