5°4 



JOHN LYON RICH 



the mountains and the plateau are valleys sometimes 10 to 15 miles 

 wide, and as much as 2,500 ft. deep (ibid., p. 622). Other observers 

 who have worked on the south side of the range report similar 

 conditions there. 



The resistant qualities of the gravel are particularly well illus- 

 trated by the Bishop conglomerate. The plateaus have remained 

 with little change while general erosion has lowered the surround- 

 ing country nearly 1,000 ft. on the average. 



In point of origin and later development, the Bishop conglomer- 

 ate is thought to represent exactly the same type of phenomenon 

 as we have described from the Silver City region ; the only difference 

 being that, in the former case, the process has been carried farther 

 and the results are just so much the more striking. 



CYCLE OF MOUNTAIN DEVELOPMENT 



If the above analysis is correct, as it £eems to be, both from the 

 theoretical side and from field observation, the influence of gravel 

 deposits is an important factor to be considered in the cycle of 

 development of mountain topography. This cycle is admittedly 

 complex, involving many factors, but for the purpose of clearly 

 presenting the point especially in mind at the present time, it is 

 not necessary to follow each of the factors involved. On the con- 

 trary, the consideration of the subject will be confined, as far as 

 practicable, to a brief outline of the manner in which gravels, by 

 reason of their selected nature, suffer less than other rocks. 



At the initiation of the cycle of mountain development let us 

 postulate the following ideal conditions: A mountain range, or 

 simple fault block of moderately resistant and varied rocks sharply 

 uplifted above the surrounding country. Free drainage from the 

 foot of the mountains to some base level, either of interior or of 

 exterior drainage, lying at a considerably lower elevation. In 

 order to give the maximum of favorable conditions, we will postu- 

 late further that the climate is semi-arid so that vegetation plays 

 a subordinate role. 



Granting these initial conditions, and assuming that there are 

 no further crustal movements, let us trace the development of 

 the mountain range. 



