GEOLOGY VICINITY OF SAN FRANCISCO. 11 



rock, I submitted a specimen from the vicinity of San Francisco to Dr. J. D. Easter, for analysis, 

 and upon it he has made the following report : 



Massive serpentine from California. 



The specimen was somewhat decomposed, of a mottled grayish-green color, and apparently 

 an aggregate of several magnesian minerals. The following is the result of a very careful 

 analysis : 



Silica 39.60 



Chromic iron 0.20 



Alumina 1.94 



Protoxide of iron and manganese 8.45 



Magnesia 36.90 



Water and loss 12.91 



100.00 



From this analysis it will be seen that this is a true serpentine, and not unlike, in composi- 

 tion, much of the serpentine of the Atlantic States. 



Sandstones and shales. — Upon the serpentine lies a d^iosit of sandstones and shales, several 

 hundred feet in thickness. They are somewhat interstratified — their strata conformable, and 

 apparently belonging to the same geological epoch, being members of a group widely spread 

 over the Pacific coast, and to which, under the name of San Francisco group, I shall frequently 

 have occasion to refer. 



The sandstone, where it has been long exposed to the action of the weather, is light brown 

 in color ; is soft and easily worked, having considerable resemblance, both in color and consist- 

 ence, to the sandstone of which the older public buildings at Washington, D. C. , are constructed. 

 From its color, and the facility with which it yields to atmospheric action, out-cropping ledges 

 of this stone closely imitate the brown and irregularly rounded masses of protruded trap, which 

 are so common in California and Oregon ; and it has often happened to me to be, at first sight, 

 deceived by the similarity. When, however, this rock has been penetrated to a considerable 

 depth, it is found to become much harder and darker, being grayish-blue in color, and 

 exceedingly dense and resistent, again resembling an erupted rock. 



No analyses have been made of these varieties to determine the nature of the chemical change 

 which gives rise to the obvious differences in physical character, but I suspect it is due to the 

 removal, by solution, of the soluble salts which it contains, and especially to the oxidation of 

 the salts of iron. It is also probable that, where it is most dense, it has in some degree experi- 

 enced the metamorphic action of the igneous rocks which it covers. Both sandstones and 

 shales generally effervesce with acids, and probably contain both lime and magnesia. 



Tli is sandstone forms the slopes of the axis lying between the Bay of San Francisco and the 

 ocean, and the rocky basis upon which the city of San Francisco rests. It is quarried near 

 water-level, at the foot of Telegraph hill ; and is found skirting the shores of the bay on both sides 

 of the entrance to the Golden Gate, as well as the strait leading into San Pablo bay. It forms 

 the greater part of the islands of Yerba Buena, Alcatraz, and, as I am informed, the western 

 declivities of the Contra Costa mountains. 



Shales. — The shales, to which I have referred, are fully exposed in the excavations made in 

 cutting the streets through the elevations which occur in the upper part of the city. They are 



