52 THE JOURNAL OF GEOLOGY. 



the often remarkable absence of fine sediments in the beds point clearly to a 

 somewhat rapid stream capable of carrying off a great deal of silt, and the 

 accumulations are probably due to rapid overloading rather than to low 

 grades of the rivers. The deep channels were filled and the gravels 

 encroached on the adjoining slopes, where they were deposited in broad 

 benches. A maximum thickness of 500 feet of deposits was attained on the 

 South Yuba, and of from 50 to 200 feet in the other parts of the lower rivers. 

 In the lower and middle Sierra some of the rivers then meandered over flood- 

 plains two or three miles wide, above which the divides of bed-rock rise to a 

 height of several hundred feet. In some instances low passes over divides 

 were covered, and temporary bifurcation and diversion of rivers into adjoin- 

 ing watersheds occurred." 



It is evident from the facts already known that at the time the 

 early gravels were deposited the northern end of the Sierra 

 Nevada was not less than 4,000 feet lower than at the present 

 time, and that its climatic circumstances as indicated by its flora 

 were not such as to give rise to either glaciers or extensive fields 

 of snow.^ For this reason it is necessary to appeal to some other 

 cause than glaciers as the source of the great mass of debris 

 deposited in the old auriferous gravel channels, and in view of the 

 facts herein cited, the writer suggests that a source may be found 

 in the large mass of residuary material upon the surface at the 

 beginning of the gravel period. There is evidence, as already 

 shown, that at the close of the Tejon disintegration exceeded 

 transportation, and residuary deposits accumulated upon the gentle 

 slopes of the land to considerable depths. This condition appears 

 to have continued during the early Miocene. The depth of dis- 

 integrated rock would vary greatly with different formations. 

 Upon the diorite and other rocks containing minerals subject to 

 ready alteration it would be deepest, and their surfaces, at least 

 in the case of the diorite, would be strewn as to-day with large 

 and small boulders of disintegration. The quartz veins which 

 intersect these rocks and the silicious slates would be but little 

 affected. The gold not enclosed in quartz veins ^ would be set 

 free. 



' See also Whitney's Auriferous Gravels, p. 295. 



-Whitney's Auriferous Gravels, p. 352. 



