GEOLOGY OF THE COAST KANGES. 



25 



Ranges proper, without its being possible at any one point to say; here the 

 Coast Ranges end and the Sierra begins. The rocks in the region northeast 

 of Klamath and Trinity rivers resemble those of the Sierra ; they are aurif- 

 erous, and often highly so. There are also deposits of gravel of considerable 

 thickness in some of the valleys, but not associated with the volcanic rocks 

 as they are in the region to be especially described in the present volume. 

 But these points may be considered more suitably in connection with the 

 description of the geological structure of the Sierra. It is sufficient, in the 

 above brief sketch of the formations of the Coast Ranges, to have indicated, 

 without attempting to go into any details, such of the prominent facts con- 

 nected with the development of that chain as are needed for an understand- 

 ing of that which follows. A resume of these, with some few additional 

 remarks, may here be appended. 



First. The Coast Ranges are made up, in by far the larger part, of sedi- 

 mentary strata of Tertiary and Cretaceous age. No rocks older than these 

 have ever been discovered. 



Second. There is no dominating or central axis of intrusive igneous rock 

 in the Coast Ranges ; but there are occasional small areas of granite, or of 

 some granitoid material, to which the sedimentary rocks in their immediate 

 neighborhood are evidently subordinated. 



Third. There are in the Coast Ranges considerable areas covered by 

 modern volcanic formations, and the activity of igneous agencies has, here 

 and there, not entirely died out, but still manifests itself in the form of hot 

 springs and solfataric emanations. The principal development of these vol- 

 canic rocks is between the Bay of San Francisco and Clear Lake, and nothing 

 of the kind has been observed to the north of the lake. 



Fourth. Of the Tertiary rocks, by far the larger portion is of Miocene age, 

 and the Pliocene is everywhere quite subordinate to it in quantity and in 

 position. The Miocene is exclusively a marine formation ; the Pliocene, on 

 the other hand, chiefly subaerial and made up of coarse gravels. There are, 

 however, some areas of Pliocene, as in San Fernando Valley and a little to 

 the south of San Francisco, which are well characterized by an abundance of 

 marine fossils. Beyond Clear Lake, to the north, there is little or no Mio- 

 cene, and the Pliocene occupies narrow strips in the bottoms of the deep 

 canons. The Eocene has not been clearly recognized in the Coast Ranges, 

 although there are rocks in the position which this formation ought to 

 occupy, but which seem to be destitute of fossils. 



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