156 GEOLOGY. 
In the bank, just behind the smelting works of the quicksilver mine, a slight exposure of the 
strata shows a very interesting flexure of the beds: 
FLEXURE OF A NEW ALMADEN. 
Although the direct transition from the unaltered sandstone and shale to these metamorphic 
strata has not been observed, there is little doubt they are of the same formation, and that the 
jaspery condition is due to igneous agencies. The chemical composition of the sandstone and the 
shales is favorable to such a result. It is regretted that analyses of each, and of the metamor- 
phosed portions cannot be presented. 
ERUPTED ROCKS—SERPENTINE. 
The extensive metamorphism, and the uplifted condition of all the strata, indicate the proximity 
of igneous rocks. They are not exposed, however, in the vicinity of San Francisco to an extent 
that the effects which have been produced would lead us to expect. The nearest exposure of 
the granitic rocks, which is known, is at the Farallones islands, twenty miles out at sea, off the 
entrance to the Golden Gate, and at Punta de los Reyes, about the same distance up the coats 
Granite may, however, occur in the mountains, south of the city, at a nearer point, but this 18 
very doubtful. It may also be found at Point Bonita. It probably forms the principal ridge 
west of New Almaden, and there is reason to believe that the sandstones and shales and the 
metamorphosed rocks rest against it. Granite is found in close proximity with the thick strata 
at San Juan, but the relations of the rocks could not be readily determined.  Trappean intru- 
sions and serpentine occur at New Almaden. 
The metamorphosed strata of Lime Point appear to be underlaid on their eastern margin by 
a dyke, or intrusion of a hard, compact trappean rock, which does not rise far above the tide- 
level; its presence might, therefore, be easily overlooked. The line of junction between the 
two formations is also indistinct, but may be traced around the base of the cliff, at a uniform 
elevation above the water, for a long distance. The position of this rock is indicated upon the 
map. Itappears to have resisted the action of the surf and currents of the channel 0 vell, 
that the degradation and undermining of the cliff has nearly ceased, it having continued until 
the softer metamorphic strata were left secure beyond the reach of the waves. The proximity 
