26 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 



Fifth. The Cretaceous rocks occupy but small areas in the southern Coast 

 Ranges. As we proceed north, we find them coming in, in force, in the 

 Monte Diablo Range, gradually widening northwards, soon forming the bulk 

 of that range, and to the north of the Bay of San Francisco becoming almost 

 the exclusive formation. It is not abundantly supplied with fossils, except 

 at certain localities of limited area. By far the larger portion of the Coast 

 Eange Cretaceous is entirely destitute of organic remains. 



These 



Sixth. While the larger portion of the Tertiary of the Coast Ranges is but 

 little metamorphosed, there has been a great deal of chemical change in the 

 Cretaceous since its deposition. The products of this metamorphism are 

 chiefly imperfect jaspers, serpentine, and mica-slates. The latter grow more 

 and more abundant as we go toward the northeastern extremity of the 

 system, beyond Clear Lake. The large amount of metamorphism by^ the 

 introduction of magnesian combinations is especially remarkable, 

 metamorphic bands and patches do not appear to follow any system in their 

 distribution ; at least, none such has hitherto been discovered. A large 

 amount of study will have to be bestowed on these rocks before their nature 



will be well understood. 



Seventh. The principal upheaval and disturbance of the Coast Range sys- 

 tem must have taken place at the close of the Miocene epoch. The Miocene 

 and the Cretaceous seem everywhere to be conformable with each other, 

 although there is a large area of the last-named formation over which it 

 does not appear that any more strata were ever deposited, and which we 

 must suppose to have been elevated above the sea-level before the Tertiary 

 beo-an to be formed. The break between the Miocene and Pliocene is very 



marked in portions of the ranges; in other parts there is a complete con- 

 formability between these two formations. This is particularly the case on 

 the inside — the eastern side — of the Coast Ranges, as is well seen in all 



the sections across the Monte Diablo Range to the San Joaquin plains. 



It follows therefore that the disturbances which took place after the depo- 

 sition of the Pliocene were somewhat local in character, and that they were 

 chiefly felt on the side of the mountains nearest the Great Valley. The 

 break at the Golden Gate, the predominance of volcanic rocks from there 

 north, for a considerable distance, and the powerful disturbances of the Plio- 

 cene in the region just south of the Bay of San Francisco may be connected 

 together as being probably the results of one general cause. 





