2IO HISTORY OF GEOLOGY AND PALAEONTOLOGY. 



Button in Colorado, and of Davis in Pennsylvania, corroborated 

 Gilbert's results. These American geologists demonstrated con- 

 clusively the backward progress of erosion during the excavation 

 of a valley, and the definite relation that exists between the 

 gradient of a river-bed and the excavating force of the river. 

 Hence the base-level of valley erosion could be ascertained with 

 great accuracy. 



Following Riitimeyer's method, W. Morris Davis depicted 

 the different stages in the development of a valley. In its 

 juvenile stage the rushing stream furrows narrow channels 

 with deep banks; in its mature stages the angle of declivity 

 is less, the valleys become broad and the banks gently sloped ; 

 in the older stage the valley-bed is worn away to the base- 

 level of denudation. Should any crust-movement locally 

 lower the base-level, then the cycle of valley-formation begins 

 anew. Davis then tried to determine the geological age of 

 various eroded plains and their drainage systems. 



The publications in Europe during the last two decades 

 of the nineteenth century are in the main based upon 

 the principles enunciated by Riitimeyer and the American 

 writers. 



A difficult problem is presented by the transverse valleys 

 that cut across mountains, plateaux, and sometimes across 

 several parallel chains. The theory of origin by tectonic faults 

 seemed especially applicable in their case, and many of the 

 best authorities at the present day support this explanation. 

 But Medlicott, in 1865, in the Memoirs of the Indian Geological 

 Survey, pointed out that not only was the central chain of the 

 Himalayas clearly older than the lateral Pliocene chains, since 

 the materials of the central chain had contributed to the rocks 

 of the lateral chains, but the Himalayan river-courses had also 

 been defined previous to the uplift of the Pliocene chains, and 

 had successfully continued to erode their valleys along the old 

 lines while these chains were being slowly uplifted. 



J. W. Powell expresses the same idea in more precise terms 

 in his explanation of the course of the Green River across the 

 Uinta Range, and of the Colorado River in its deep cutting 

 through the Arizona plateaux. In both cases the river passes 

 from younger strata into older; and Powell's explanation of 

 the apparent enigma is that after the river had eroded its 

 channel rocks were uplifted at one portion of its course, but 

 so slow was the rate of uplift that the river was enabled to 



