

L 



l JIiW^^^Hh^^^^DIII 



22 



THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 











largely developed, forming much the larger portion of the high and rough 

 mountain region lying between Pacheco's and the Livermore passes, of which 

 the culminating point is Mount Hamilton (4,448 feet). This central mass 

 of highly altered Cretaceous is flanked on both sides with Tertiary, and in 

 the division of the Monte Diablo group, which lies north of Livermore Pass, 

 and extends thence to the Straits of Carquines, there is a large area occu- 

 pied by Tertiary, the central and dominating mass of Monte Diablo itself ? 

 however, being made up of Cretaceous. North of Pacheco's Pass there are 

 only a few patches of volcanic rock, and there is no granite in this range. 

 On the opposite side of the bay, in the Santa Cruz Range, there are a few 

 very limited areas of volcanic, but much larger ones of granitic or granitoid 

 rock, wdiich protrude through the Miocene strata in such a way as to show 

 that the latter have been considerably disturbed and lifted up by the in- 

 trusion of the eruptive material. 



Along the flanks of the Miocene near San Jose Mission, and in Alameda 

 Canon, there are occasional quite large masses of gravel, lying nearly hori- 

 zontally on the upturned edge of the Miocene. Similar deposits, but of more 

 limited extent, are also found at various points on the eastern edge of the 

 Sante Cruz Range. These gravels near Mission San Jose contain bones of 

 elephant, mastodon, and lama, as also strata filled w r ith fresh-water shells. 

 The more extensive beds of similar character south of Livermore Valley 

 have thus far proved to be destitute of fossils. They are developed to a 



■ 



thickness of 200 feet or more, and are supposed to be of later Pliocene age. 

 These gravels have never been productive of gold, so far as known to the 

 w T riter. Some further remarks on these Coast Range gravels will be intro- 

 duced in a future chapter. 



North of the Bay of San Francisco the same formations occur which have 

 been already noticed as making up the mass of the Coast Ranges to the 

 south; but the Cretaceous becomes more and more prominent and the Ter- 

 tiary almost disappears after we get to the north of Clear Lake. From San 

 Pablo Bay north to that lake there is a large amount of volcanic material, 

 and abundant evidence of the quite recent activity of igneous agencies. This 

 volcanic band extends from the bay north through the centre of the ranges 

 in numerous large and small isolated fields, so intricately distributed over 

 the underlying metamorphic Cretaceous rocks, that it would require an accu- 

 rate map on a large scale, and much research to be able to lay them down 

 correctly. The high points of Mount Helena (3,343 feet), Cobb Mountain, 























