10 GEOLOGY VICINITY OF SAN FRANCISCO. 



bay the middle, and the western half of San Pablo bay and Sonoma valley the northern por 

 tion. Towards its southern extremity it is enclosed between the San Francisco or Coast Range 

 and the Diablo mountains, with which the Contra Costa range here unites. Near its northern 

 end the Contra Costa range may be said to terminate, permitting San Pablo bay to stretch 

 eastward to the base of the Diablo mountains, thus adding the eastern half of this bay and Napa 

 valley to its area. 



The subordinate axes which traverse this valley have produced the island of Yerba Buena, the 

 east and west shores of the straits connecting San Francisco and San Pablo bays, Point San 

 Pedro, &c. Through the most westerly of these axes the Golden Gate is opened as a narrow 

 strait, of great depth, and bounded by rocky, and in some places, precipitous walls. Through 

 the most easterly the straits of Carquines pass, presenting the same general characters. 



LOCAL GEOLOGY. 



San Francisco range. This range, in the vicinity of the city of San Francisco, forms low 

 mountains or hills, none of which exceed 2,000 feet in height. They are composed of shales, 

 sandstones, serpentine, and trap. 



Trap. This forms the lowest and central portion of the range, where it is cut by the Golden 

 Gate, and is only exposed low down in the cliff which forms the north wall of this strait. 



A few miles further north, in the same range, trap has been poured out in abundance, but it 

 has apparently not burst through the overlying strata anywhere in the vicinity of San Fran 

 cisco. The trap of the Golden Gate is dark brown in color, and more compact than that which 

 has reached the surface in the vicinity of Petaluma. 



Serpentine. Overlying the trap on the north side of the gate, and on the south side forming 

 the axis and nucleus of the chain, are heavy beds or masses of serpentine. Here, as wherever 

 found in the vicinity of San Francisco, it is grayish green in color, and varies considerably in 

 hardness and texture ; the weathered portion being rendered friable by numerous joints, of which 

 the surfaces are more or less covered with a white, probably magnesian, stain. At Fort 

 Point, the best exposure of this rock which I saw, some portions of it are very compact and 

 homogeneous, while others are somewhat foliated. The position which it occupies, and the 

 relations which it sustains to the accompanying strata, seemed to me to indicate that it is an 

 erupted rock. It forms the centre of the ridge, bearing on either side the inclined and convo 

 luted strata of sandstones and shales, which cover and embrace it. 



It is, perhaps, possible that it is a metamorphosed form of one of the group of stratified depo 

 sits with which it is associated, but the sandstones and shales, fossiliferous or barren, which, 

 with trap, serpentine, and granite, go to make up the mass of the Coast Mountains, are scarcely 

 capable of assuming this form under any phase of metamorphic action with which we are 

 familiar ; nor is there a dolomite or other magnesian rock on the western coast, which might 

 be supposed to exhibit the unchanged state of the serpentine. 



My own observations would therefore lead me to consider it an intruded rock, whatever 

 inferences as to the origin of serpentine might be drawn from other localities, till new facts 

 shall be brought to light, which will offer a more plausible explanation of the phenomena. 



As the origin and composition of serpentines have recently afforded interesting subjects of 

 inquiry to geologists and chemists, and as it has been suggested that the serpentine of California, 

 as has been proved of some of the eastern (so called) serpentines, was, perhaps, not a magnesian 



