1918] Davis: The Radiolarian Cherts of the Franciscan Group 363 



to account for the presence of lenses of chert within a sandstone 

 which could not have been deposited far below sea level. 



Possibilities which might reconcile the hypothesis of abyssal origin 

 with the known facts. — In the case of the Franciscan cherts it might 

 be possible to explain the alternation of the two main horizons of 

 chert with coarse sandstone without involving extreme movements of 

 the earth's crust. One might imagine a land mass somewhere to the 

 east of the present Great Valley of California. Another mountain 

 range with granitic rocks exposed to erosion is postulated on the site 

 of the present Coast Ranges, or standing a little to the west of them. 

 From the western mountain mass, it is conceivable that there might be 

 a rapid depression toward the present site of the Great Valley, so that 

 a narrow body of water of considerable depth existed there. From 

 the Coast Range mountains, coarse sediments would be carried down 

 into the deep water near the west side of the present valley. Then, 

 during fluctuations of the level of the Coast Range province, which 

 appears to have always been geologically unstable, there might have 

 been times when the Coast Range was submerged. During these times 

 no terrigenous sediment would reach the basin of accumulation save 

 fine materials carried out from the land mass to the east. It might 

 be possible in such a depression to get alternations of red shale with 

 radiolarian ooze, during times of Coast Range submergence. At a 

 later time the uplift of the region would cause a renewal of coarse 

 sedimentation and the radiolarian oozes would be covered by sand- 

 stone. 



Another possibility is that there was a large river carrying ma- 

 terial and dropping it into an ocean deep not far from land. This 

 river might have built up a delta in the shallower water near shore. 

 Rivers change on their deltas, and occasionally these changes are very 

 great, as for example the changes of the Hoang Ho River in China. 13 " 

 During the time of diversion of the river, there would result an 

 accumulation of non-terrigenous sediment in the deeper parts of the 

 basin. In this way it would be possible to get notable alternations in 

 the character of the sediment without any great changes in the level 

 of the land or in the relations of land and sea. 



Opposed to both these explanations of the alternation of sandstone 

 with cherts, is the evidence which goes to prove that the typical Fran- 

 ciscan sandstone is a continental deposit. If this is correct it would 

 require that the region undergo an absolute change of level of at least 



130 Tarr and Martin, College Physiography, p. 160, New York, 1914. 



