34 



University of California Publications in Geology [ VoL 11 



Ocean. 53 It would act as the present Coast Ranges, which, in a 

 measure, screen the west side of the San Joaquin Valley from precipi- 

 tation. The conditions would be analogous to those in Patagonia. 

 The plains of Patagonia are semiarid and barren, due to the fact that 

 moisture laden winds from the Pacific Ocean lose their moisture in 

 passing over the Andes and descend upon the plains to the east as 

 drying winds. 54 



On the arid piedmont plains, there would be deposited the materi- 

 als brought down from the mountains to the west. In a general way, 

 the conditions would be somewhat like the deposition of material by 

 rivers of the peninsular region of India, or analogous to the deposition 

 of material by aggrading streams on the Great Plains at the eastern 

 base of the Rocky Mountains, or on the plains of southern Argentina. 

 In the high mountains to the west mechanical disintegration of rocks 

 would predominate. Large amounts of granitic sand would be pro- 

 duced there and from time to time swept down in flooded streams 

 during occasional torrential rains. The material would be deposited 

 on the gentler slopes at the base of the mountains and, on its consoli- 

 dation, would give rise to a sandstone of the Franciscan type. The 

 angular character of the grains and the heterogeneous nature of the 

 rock, due to very poor sorting of the component minerals, are both 

 accordant with such a picture. Under the conditions postulated, the 

 feldspathic sand would be carried down in large amounts in heavily 

 loaded streams in which sorting and abrasion would be at a minimum. 

 Between times of torrential rains the normal drainage of the moun- 

 tains would be by streams which would run down in occasional chan- 

 nels across the piedmont slope. 



MAEINE DEPOSITION 



As far as the petrographic evidence goes it is possible that such a 

 formation as the Franciscan sandstone might have been deposited 

 beneath the sea, the sand being carried down during torrential rains 

 and covered quickly. If it had been supplied gradually in small 

 amounts and worked over by the waves on beaches, the greater part of 

 the feldspar would have been disintegrated and the residual quartz 

 grains would have become rounded. 



53 The assumption is here made of course that the prevailing winds were 

 from the west as at the present time; also with differences in the path of storms 

 there might result an arid coastal belt. 



54 Hatcher, J. B., Narrative of the Princeton Patagonia Expedition, vol. 1, 

 p. 216, 1903. 



