282 GEOLOGY. 
the whole appearance was changed, and clean, sound stone could be procured. This rock has 
also been used in small cubical blocks for paving the streets. "The rock at Point Reyes is darker 
and more hard than that of Monterey, and contains a little hornblende. l 
The quarries are directly upon the seashore, and the stone can be placed on shipboard without 
previous land transportation. Their proximity to San Francisco, and the facility with which 
the stone can be obtained, renders them exceedingly valuable as a source of building material. 
Mormon Island.— American River.—A very beautiful and compact syenitic granite is found at 
Mormon Island. It has been used in the construction of buildings in Sacramento. It is, how- 
ever, twenty miles from water transportation, and, consequently, cannot be obtained with 
economy. 
SANDSTONE. _ 
The San Francisco sandstone is used to a considerable extent for buildings in the city, and 
for foundations where ‘‘ dimension stone"' are not required. The rock, however, is not obtained 
from the outcrops in the city ; it is more favorably exposed for quarrying at other points around 
the head-lands of the bay and on the islands. Quarries have been opened at Yerba Buena 
Island, opposite the city, at Angel Island, and at the State’s Prison. The latter quarry is 
most extensively worked, and it supplies a large amount of dimension stone. 
The color of this sandstone varies at the different localities, and its weathered portions are 
much rusted and stained by the formation of peroxide of iron. In general, the color of the 
undecomposed, or not oxidized, portions is a dark, bluish-green, very much like that of some of 
the trappean rocks. Wherever it is quarried into it is found that the outer or exposed portions 
have become rusted and softened to a great depth, and that it is traversed by innumerable 
fissures like cleavage planes, which prevent large and solid rectangular blocks from being 
obtained. It has been found necessary to excavate below this thick and partly decomposed cov- 
ering, and even to quarry below tide-water, in order to procure stone of good size and uniform 
in texture and color. The quality of the stone below tide is superior to that above, in every 
respect, and it appears to be the only portion of the stone in its normal or unchanged condition. 
From the fact that this sandstone has undergone a change of color, and is partly decomposed to 
a great depth, even to the level of tide water in most places, we are led to expect that the 
unchanged portions, when exposed to similar conditions of air and moisture, will undergo & 
corresponding change oftexture. It is, however, impossible to say how long the stone may 
remain in a wall without undergoing a perceptible change. It would doubtless be more durable 
in a dry wall than in one kept moist by the absorption of moisture from adjoining banks of 
earth. 
This sandstone contains a large per centage of oxide of iron, which probably exists as à prot- 
oxide in the undecomposed portions, and as the sesquioxide in those portions which have been 
exposed to the weather; the change thus producing the brown or drab color seen at all of the 
outerops. The rock, in some places, also contains very minute grains of sulphuret of iron, but 
this is not a common ingredient. Carbonate of lime exists in all of the specimens that I have 
examined, and the rock is, in fact, a calcareous sandstone. : " 
It is a very firm and dense rock, and is very strong and tough. At first sight it is -— 
mistaken for trap, especially at the weathered outcrops, where the stratification is not distinctly 
e 
The stone obtained from the State's Prison quarry is lighter in its color than that from Yerba 
