12 GEOLOGY — VICINITY OF SAN FKANCISCO. 



greenish or yellowish-brown in color, and contain varying proportions of clay and sand. Where 

 laid open in the upper part of the city, they are very friable and easily removed ; but in 

 localities where less affected by the action of the air, they are considerably more compact, and 

 closely resemble some of the older clay slates. 



I was not able to detect in the immediate vicinity of San Francisco, either in the sandstone 

 or shale, any other fossils than small particles of carbonized vegetable matter ; but on San 

 Pablo bay this group is highly fossiliferous, and, on the Pacific side of the San Francisco axis, 

 great numbers of an extinct species of Sculella {S. interlineata) are washed out by the waves 

 from a sandstone similar, in its lithological characters, to that underlying the city. I have 

 little doubt that the sandstones which flank the serpentine axes of southern California, and 

 which contain great numbers of ScuteUae, Ostreae, Pectens, and other shells, regarded by Mr. 

 Conrad as Miocene, are of the same age, and, perhaps, continuous with the sandstones and 

 shales of San Francisco. 



The sandstone which I have described is the only rock which will furnish a material suitable 

 for architectural purposes in the immediate neighborhood of this city. As a building stone it 

 does not rank high. The softer portions, though easily worked, are too friable to retain any 

 ornament or inscription, or to resist the crushing force of great weight; while that which is 

 quarried from a greater depth, though hard, tough, and handsome, when first taken out, will 

 be liable to fade, and, probably, to a comparatively rapid decomposition. The demand for a 

 good building material which now exists in the city, and which will hereafter be more sensibly 

 felt, can, however, be fully supplied from the stores of granite, porphyry, trachyte, and trap, 

 which are to be found in the Coast Ranges and Sierra Nevada in the greatest abundance, and at 

 points neither remote nor inaccessible. 



Jasper. — In a great number of localities in the vicinity of San Francisco, ridges and masses 

 of red jaspery rock are seen. It crops out in the vicinity of the Presidio, on the south side of the 

 Golden Gate, and exists in large quanties at Point Diablo, and thence to Saucilito, on the 

 north side. It is also found in Raccoon straits, and on the southwest side of Angels' Island 

 forms Red Rock, and also occurs at various points further northward. It frequently occurs in 

 ridges, having the appearance of an erupted rock, protruded along lines of upheaval. It is red, 

 yellow, or green in color, but oftenest blood-red, or some intermediated shade between that and 

 pink, being usually somewhat mottled, clouded, or striped. 



Veins of white quartz, generally small, traverse it in every direction, and, where it is 

 weathered, it is often peculiarly cellular, ragged, and rough. Where stratified, the laminae 

 which it exhibits are twisted and contorted in all possible directions, and whatever is the 

 history of the material of which it is composed, whether it is thrown up from below or, as 

 is more probable, it is a metamorphosed form of the associated rocks, it is evident that it has 

 been subjected to a high degree of heat. These jaspery rocks are, equally with the serpentines, 

 a marked feature of the geology of the Coast Ranges, from the Gulf of California to the 

 Columbia. 



Surface geology. — The hills about San Francisco are covered with loose and, in some places, 

 drifting sand, which has, apparently, in greater part, been derived from the shore of the Pacific, 

 whence it has been driven by the strong and ever-blowing westerly winds. Along the shore of 

 the bay, in many places, alluvial deposits, consisting of sands and clays containing vegetable 

 matter, have collected to a considerable depth. They have probably been formed by the wash- 

 ing down of the higher grounds, and belong altogether to the present epoch. 



