158 
GEOLOGY. 
Asylum ; but there is nothing to recommend it as a building stone, except, in this instance, its 
presence upon the spot. It is not well calculated to resist the action of the weather. 
It is very probable that the higher portions of Mount Diablo are formed of a similar serpen¬ 
tine. A large quantity of rock was once quarried there and taken away, under the suppo¬ 
sition that it contained gold. A fragment of this was obtained ; it proves to he a mass of 
diallage or bronzite in rough crystals, with the usual metalloidal lustre. It closely resembles 
the bronzite of Bacher Mount, Styria, which occurs with serpentine. 
The ridge of compact serpentinoid rock at Fort Point appears to he eruptive, and to he more 
recent than the sandstone of the hay of San Francisco. It appeals probable, also, that the strata 
of sandstone and shale were much uplifted or disturbed by its intrusion, although the great flexures 
of the strata, and perhaps the principal metamorphoses, were, in all probability, attendant upon 
the formation of a granitic axis of considerable extent, hut which, in the vicinity of San Fran¬ 
cisco and the Golden Gate, is submerged in the waters of the Pacific. That the strata were 
disturbed by the serpentine, is indicated by the relative positions of the rocks, as seen in the 
exposures along the shore of the Golden Gate from San Francisco to Point Lohos. These 
positions are shown on the little section, page 155, which is intended to represent the succession 
of the formations along the line A, B, upon the map, and the probable flexures of the strata. 
The sandstone is found upon both sides of the serpentine, ar.d appears to rest upon it on the 
east, and to underlie it on the west. An outcrop of metamorphosed rock is found near the line 
of junction on the east side, hut on the shore near the beach it is not visible. At this point, 
however, we find a body of sandstone strata, intercalated or imbedded in the ser^ entine, as 
represented in the annexed figure. 
SANDSTONE AND SHALES IMBEDDED IN SERPENTINE. 
Serpentine. Sandstone. Serpentine. 
This mass of strata is between two and three hundred feet thick; the beds dip eastward at 
an angle of about 75°. The series consists principally of slates and shales, but there are several 
beds of compact sandstone from two to six feet thick. The relative position of the strata and 
the serpentine indicates that at the time of its inruption it probably followed the planes of 
stratification for the greater part of its course. These beds of sandstone and shale are much 
harder and more compact than the strata of the same formation at a distance from the intrusive 
rock ; but the color and mineral characters are well preserved, and they do not show any signs 
of change from igneous action. 
The line of contact of the serpentine with the outlying beds of sandstone on the west side of 
the point was not seen, but the character of the surface on that side indicates that the strata dip 
under the serpentine. The valley through which the little brook, leading from the lake, flows 
is coincident with the line of junction between the two formations. 
Serpentine is found to outcrop, in connexion with sandstone strata, similar to those of San 
Francisco, in the hills between the southern end of Tomales Bay and the Mission of San Rafael. 
It is also reported to be an abundant rock throughout the Coast Mountains. Wherever it came 
under my observation it presented the aspect of an intrusive rock, and I have so regarded it. 
There is little doubt that the outcrops have undergone great changes, even to a great depth, by 
the action of percolating water and the atmosphere. 
