192 abstracts: geology 



taceous and Tertiary time. Fluctuations of the western shore line 

 caused the streams at times to extend far into the areas now occupied 

 by the Sacramento and San Joaquin valleys and. at times to debauch 

 upon flood planes high on the flanks of the range. Faulting movements 

 with downthrow on the east side, probably begin ning in Cretaceous 

 time, had transformed an approximately symmetrical range into a mono- 

 clinal one with steep easterly slope. Gradually the mountains were 

 worn down, the rivers flowed from low divides, meandering among 

 longitudinal ridges, and the whole slope was covered by the dense vege- 

 tation of a damp semitropical climate. 



Toward the end of Tertiary time rhyolite flows filled the valleys, 

 covered the auriferous gravels, and new stream courses were outlined. 

 Renewed disturbance began along the scarcely healed eastern breaks, 

 resulting in a westward tilting of the main blocks, combined with normal 

 faulting and subsidence of the blocks in western Nevada. In conse- 

 quence of this disturbance the monoclinal nature of the range became 

 strongly emphasized, and the streams immediately began to cut their 

 beds deeper. At the end of the Tertiary, eruptions of andesites and their 

 tuffs and breccias began in enormous volume and filled the Tertiary 

 valleys to the rims. During the subsequent canyon-cutting epoch 

 which belongs in part at least to the Quaternary period, the erosion has 

 laid bare the old gravels and in most cases completely intersected the 

 old valleys. 



The first part of the book outlines the history of the valley border and 

 describes the sedimentary and volcanic formations exposed at the west- 

 ern base of the range. The Tertiary drainage system and the pre- 

 volcanic surface is then described. The courses of the main rivers of 

 Tertiary age draining the northern part of the range are traced in detail. 

 Toward the southern part of the area the present drainage corresponds 

 fairly well with that of the Tertiary period, the principal difference 

 appearing to be that many of the Tertiary rivers flowed parallel with the 

 range in longitudinal valleys which have been in most cases squarely 

 intersected by the present Quaternary rivers. Feather River, one of 

 the greatest of the present water courses, had no adequate representative 

 in Tertiary time, but that part of the slope now belonging to it drained 

 to the north. 



Evidence is adduced by numerous profiles and sections of the charac- 

 ter of the Tertiary topography, and of the present grade of the Tertiary 

 stream channels. These profiles, by the infallible relation between 

 extremely steep transverse grades and extremely flat longitudinal grades, 



