1 



GEOLOGY OF THE COAST RANGES. 



21 



Monte Diablo Kange is flanked by Miocene and Pliocene Tertiary on its 

 eastern edge, the whole series having a dip towards the San Joaquin Valley, 

 steep in the centre of the range, and gradually diminishing as we go east- 

 ward. Between the Gavilan and the Monte Diablo Range there is a heavy 

 mass of gravel, of Pliocene age, which is distinctly stratified and turned up 

 into an almost vertical position. All this portion of the Coast Ranges ex- 

 hibits abundant evidence of very recent disturbances, lasting through the 

 Pliocene epoch. Volcanic rocks are not found in large quantities anywhere in 

 the Coast Ranges south of the Bay of San Francisco. There is, however, a 

 line of protrusions of trachyte, extending from a point a little south of the 

 west end of Pacheco's Pass to the southeast, diagonally across the Monte 

 Diablo Range, as also some small patches of eruptive material at various 

 points near San Luis Obispo. The granite of the Santa Lucia Range has not 

 been sufficiently investigated to allow of an opinion being formed as to its 



stratigraphical relations. 



North of Monterey Bay, in the region adjacent to the Bay of San Fran- 

 cisco, the formations have been studied with more care than has been possible 

 farther south.* There are two well-marked ranges in this portion of the 



Coast Mountains. One — 



of Monterey up through the Peninsula of San Francisco ; the other 

 Monte Diablo Range — is separated from the first named by the depression 

 of the Santa Clara Vallev, which is continued north in the Bay of San Fran- 



the Santa Cruz Range — extends from the Bay 



— the 



cisco. The Santa Cruz Range is largely made up of little altered Miocene 

 strata, similar to those described as occurring farther south. But there is 

 on its eastern edge, as it is prolonged through the Peninsula, a belt of Cre- 

 taceous rocks, forming the higher portion of the divide, which culminates in 

 Mount Bache (3,790 feet high). These rocks are greatly changed from their 

 original character by chemical action ; and they are largely made of serpen- 

 tine and an imperfect jasper, locally known as u red rock." It is this forma- 

 tion which carries the quicksilver ores so extensively worked in the vicinity 

 °f the Bay of San Francisco. This Cretaceous formation has also a narrow 

 belt of limestone running through it, a rock not frequently occurring in 

 California, These rocks are almost entirely destitute of fossils, although 

 enough have been found to fix the geological age of the formation. 



On the opposite side of the Santa Clara Valley the Cretaceous strata are 



* 



A geological map of the region adjacent to the Bay of San Francisco, on a scale of two miles to an 

 lnc '», has been prepared for publication. 



