1 66 POPULAR SCIENCE MONTHLY. 



of ordinary valleys by their rivers, maintained (1886), with the results 

 of our western surveys before him, that fissures were probably re- 

 sponsible for the origin of the deep and narrow canyons of the Colo- 

 rado plateau. 



The tumultuous forms of lofty mountains 'tossed up' as they seem 

 to be when viewed from some commanding height, are, in by far the 

 greater number of examples yet studied, undoubtedly the result of the 

 slow erosion of the valleys between them; but it should not be for- 

 gotten that regions of very recent disturbances — as the earth counts 

 time- -may possess strong inequalities directly due to deformation. 

 The tilted lava blocks of Oregon have already been mentioned. The 

 bold forms of the St. Elias Alps, also described by Russell, are regarded 

 by him as chiefly produced by the tilting of huge crustal blocks on 

 which erosion has as yet done relatively little work. An altogether 

 exceptional case is described by Button, who says that on the margin 

 of one of the "high plateaus of Utah a huge block seems to have 

 cracked off and rolled over, the beds opening with a V and forming a 

 valley of grand dimensions." 'Kift valleys,' or trough-like depressions 

 produced by the down-faulting of long, narrow, crustal blocks with re- 

 spect to the bordering masses, are occasionally found, as in eastern 

 Africa, where the 'Great Rift valley' has been described by Gregory. 

 Trough-like depressions of similar origin, but much more affected by 

 the degradation of their borders and the aggradation of their floors, 

 are known to European geographers in the valleys of the Saone and of 

 the middle Rhine. But no rift valley, no depression between the tilted 

 lava blocks, resembles the branching valleys that are produced by the 

 erosive action of running water. 



Thus far, while much attention had been given to the w?>rk of rivers, 

 little or no attention had been given to the arrangement of their 

 courses. It seems to have been tacitly assumed that the courses of all 

 streams were consequent upon the slope of the initial land surface. 

 The explicit recognition of this origin, indicated by the provision of a 

 special name, 'consequent streams,' was an important step in advance 

 due to our western geologists. The discovery soon followed that rivers 

 have held their courses through mountain ridges that slowly rose across 

 their path; the rivers, concentrating the drainage of a large headwater 

 region upon a narrow line, cut down their channels as the land was 

 raised. This idea first came into prominence through Powell's report 

 on the Colorado River of the West (1875), in which he gave the name, 

 'antecedent,' to rivers of this class. He believed that the Green river, 

 in its passage through the Uinta mountains, was to be explained as an 

 antecedent stream. Much doubt has, however, been thrown upon this 

 interpretation. Other accounts of antecedent rivers have been pub- 

 lished, and to-day the Green is not so safe a type of antecedence as 



