148 
GEOLOGY. 
be regarded as, in general, applicable to the unchanged portion of the rock at San Francisco, 
and at other points about to be mentioned. The sandstones at Point San Josef and those of 
Telegraph Hill are traversed by thin and irregular seams of quartz, running in various direc¬ 
tions. These may have had their origin at the time of the intrusion of neighboring igneous 
rocks, being deposited along the sides of slight cracks and fissures by escaping hot vapors. 
Veins of carbonate of lime, nearly the same in size and appearance as those of quartz, aue also 
found traversing this sandstone in some places, but the two minerals were not found together. 
Point Lobos—the outer headland of the Golden Gate on the south side—is likewise of sand¬ 
stone, similar to that of San Francisco. The continued action of the ocean swell has worn the 
rocks into rugged cliffs and excavated caverns and arches. Many large masses are detached 
from the cliff, and lie scattered about in the surf. These isolated, island rocks are the places of 
resort for sea-birds and the huge “sea-lion.” 
The direction and dip of these strata are not very distinct at this point. The greater part of 
the formation is hidden from view by an immense deposit of blown sand, and the surf prevents 
any extended examination under the cliffs. A local trend of a few degrees north of west was, 
however, observed. On the opposite side, or north shore of the channel, the prevailing dip is 
westward, and it is . more than probable that the rocks of Point Lobos have the same direction.. 
These strata extend eastwardly as far as a little brook that descends from the Mountain lake, 
and empties into the channel. 
Excavations for building-stone have been made on the southeast end of Angel island, which 
bears northwest from Yerba Buena, and is in the range of the strata. It is composed of sand¬ 
stone, similar to that on Yerba Buena island, but is not so dark in color or so hard. Specimens 
which I have examined contain a notable quantity of carbonate of lime. The strata dip west- 
wardly, and the quarry is opened on their upturned edges, and not at the ends of the beds, as at 
Yerba Buena. The weathered surfaces of the strata present the same rusty color as those at 
San Francisco and Yerba Buena, and the divisional planes or cleavages are numerous. 1 
Another extensive outcrop of sandstone, and a quarry, is found at the State’s Prison, on Point 
San Quentin. It is worked, by convicts, to a greater depth than either of the other places that 
have been described. It is the same sandstone formation, and furnishes a large quantity of 
good “blue stone.” The excavation (in 1854) had extended to a depth of about thirty feet 
below high-water mark ; at that depth the blocks of stone are larger, and without the rusty 
color attendant on surface decomposition. The trend of the strata is nearly north 50° west; the 
dip southwest at variable angles, ranging from 45 to 55 degrees. The lay of the strata and the 
general outline of the quarry is shown in the annexed section; the horizontal line on the left 
representing, nearly, the height of the waters of the bay. 
The strata rise into a slight hill on the east side of the opening, and soon loose the compact, 
massive character, becoming thinner and more broken, and then pass into a thick body of 
argillaceous shales. 
The operation of quarrying below the surface is, necessarily, more expensive than excavating 
1 I have represented Angel island on the map as wholly formed of sandstone and shales, although I did not visit the 
western portions. It has, however, the appearance of the sandstone. Lieutenant Belcher, it appears, collected specimens 
of serpentine on the western side, and the occurrence of jasper rock is noted. It is further stated that the opposite shores 
of the main land, or promontory on the north, afford specimens of actynolite, mica slate, and talc slate .—Zoology of Captain 
Beechey’s Voyage: Geology , p. 175. It is possible that fragments of the slaty serpentine are here mentioned as talc slate, and the 
mica slate may have been a transported mass, for it is not probable that the true micaceous slates occur at that point. 
