262 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



I. The Pre-Pleistocene Orogenic History of the 



Sierra Nevada. 



In the Sierra Nevada, folding and lateral thrust move- 

 ments appear to have culminated about the close of the 

 Jurassic, and this may likewise have been the case in the 

 Great Basin. Professor Whitney^ regarded this time as pre- 

 eminently the mountain-building epoch of the region west 

 of the Wasatch. The Sierra may thus be said to have first 

 existed as a great mountain range in early Cretaceous 

 time. This Jurassic range, as well as the Jurassic and 

 older ranges of the Great Basin, was greatly eroded dur- 

 ing Cretaceous and Tertiary time; and we may then pict- 

 ure this entire region from the Pacific Ocean eastward as 

 being in middle Tertiary time characterized by low ridges 

 with broad basins often occupied by extensive lakes. ^ 

 It should here be noted, however, that some of these 

 basins may have been formed by subsidence along nor- 

 mal faults in early Tertiary time, as suggested by King 

 and Lindgren. The orogenic disturbances of post-Miocene 

 time have displaced these Tertiary lake beds at many 

 places. Considerable areas have been eroded, and still 

 larger areas are covered by Pleistocene deposits. Patches 

 of them, and often extensive areas are, however, still to be 

 found. Examples of such deposits are the Pah-Ute beds 

 of north-central Oregon; the Mohawk Valley and Boca- 

 Truckee Valley beds of the eastern Sierra Nevada; the 

 Tertiary beds, mainly of volcanic detritus, of Carson Val- 

 ley;^ the Eocene or Miocene beds (Esmeralda Formation)* 

 of Esmeralda County, Nevada; and the Tertiary beds of 

 the Mojave and Colorado deserts. 



There appears to be a good basis for the hypothesis of 

 Le Conte, that the whole region from the Wasatch to the 

 Sierra was uplifted into a great arch, and that the valleys 



1 Auriferous Gravels, iSSo, p. 315. 



-See Whitney's Climatic Changes, 18S2, pp. 114-115. 



sjourn. Geol., Vol. IV, 1896, p. 900. 



■* Amer. Geol., Vol. XXV, 1900, pp. 168-170. 



