GEOLOGY OF THE COAST RANGES. 21 
Monte Diablo Range 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 opimion 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 — the Santa Cruz Range — extends from the Bay 
of Monterey up through the Peninsula of San Francisco ; the other — the 
Monte Diablo Range —is separated from the first named by the depression 
of the Santa Clara Valley, which is continued north in the Bay of San Fran- 
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 culmmates 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 “red rock.” ~ It is this forma- 
tion which carries the quicksilver ores so extensively worked in the vicinity 
of 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 
inch, has been prepared for publication. 
