Page:The Journal of geology (IA journalofgeology21894univers).pdf/66
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 diversions of rivers into adjoining 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.[1] 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 disintegrated 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[2] would be set free.