CHAPTER IV. 



THE MASSIVE ROCKS. 



General character of the massive rocks. The litliology of tllG Pticific slopC liaS 



received so much attention of late years tliat it is unnecessary and would 

 be undesirable to treat the eruptive rocks of this area as if they were 

 undescribed. The region is indeed vast; but it is also one in which the 

 character of the rocks is remarkably persistent. In the following pages, 

 therefore, only the more peculiar features of the massive rocks encountered 

 in the present investigation will be enlarged upon. These are granite, 

 older porphyries, andesites of several varieties, rhyolite, and basalt. 



PRK-TERTIARY ERUPTIVES. 



Distribution of the granite. — As luis bccn statcd iu the prccediug chapter, 

 granite appears to underlie the sedimentary rocks of the Coast Ranges and 

 of the Sierra Nevada. According to Professor Whitney, a large portion of 

 the mountain system from Fort Tejon southward is composed of granite. 

 There are also large exposures of it in Shasta and Trinity Counties. In the 

 region between Clear Lake and New Idria it is found in the Gavilan Range, 

 occupies considerable areas near Monterey, forms the Farallone Islands, 

 and appears at Point Reyes and other localities in the neighborhood. Near 

 the town of Guadala, on the coast, in Mendocino County, large masses of 

 conglomerate are formed of granite bowlders cemented by granitic detritus. 

 In the interior of the Coast Ranges north of San Francisco it has not been 

 met with in place to the south of the Trinity Mountains, but probably 

 occurs in some of the chaparral-covered hills, since a very large part of the 

 pebbles in Cache Creek are granite. 



