SERPENTINE OF FORT POINT, ITS LITHOLOGICAL CHARACTERS. 157 
and direct contact of this rock of igneous origiu, with a portion of the jaspery strata, indicate 
that it has caused their metamorphosis. It is, however, difficult to conceive that such extended 
effects were produced by this rock, which has such a limited exposure at the surface, unless we 
conclude that it has a broad subterranean extension. 
The formation next in importance to the sandstone, in point of extent and development in the 
vicinity of San Francisco, is the serpentine, or serpentinoid rock, of Fort Point. It forms a 
high and prominent ridge about midway between the shore of the Pacific and the Bay of San 
Francisco, and extends in a northwest and southeast direction, abutting upon the Golden Gate 
and forming Fort Point. The width of the ridge is between one and two miles, but its extension 
southward is not accurately known. It is partly obscured in that direction by sand, but forms 
a knob at the Orphan Asylum, near the Mission. It is bounded on both sides by the San Fran- 
cisco sandstone, and its end, at the Golden Gate, forms a bold bluff facing the Pacific. Fort 
Point is its extreme northern point, and formerly presented a high bluff with a flat table-like 
summit, upon which the Mexican fort was built.! This has recontly been cut away to prepare 
for the erection of new fortifications. This cutting down of such a large mass of the rock 
exposed a broad surface of the interior, and permits its structure to be conveniently studied. 
The rock, in its appearance, is unlike the serpentine of Hoboken, New Jersey, and Staten Island, 
New York; nor is it like the serpentine of Milford and New Haven, Connecticut; being of a 
darker color, harder, and filled with distinct crystals of diallage, from one-eighth to one-quarter 
and half an inch in length. These present brilliant cleavage surfaces when a mass of the rock 
is broken up. The fracture of some of the dark portions of the rock is sub-conchoidal, the 
resulting surfaces being smooth, and, in this respect, differing from the serpentines mentioned. 
Another, and a prominent peculiarity of the rock is its globular character, it being made up of 
nearly spherical, boulder-like masses, included in a shaly or slaty portion that readily splits up 
and falls into fragments on exposure to the air. These slaty portions present the common 
smoothed or furrowed appearance sccetimes seen on the surfaces of both igneous and sedimen- 
tary rocks where they have been rubbed together, under great pressure, as in the walls of mineral 
veins. It fills all the space between the masses, and gives them the appearance of having been 
coated with a soft plastic cement. The color of these globular masses is dark olive-green; they 
are very hard and compact, and not only contain the crystals of diallage, but in some instances 
are traversed in every direction by thin veins of amianthus or chrysotile, with the fibres trans- 
verse to the walls. These seams are generally very thin, and, by intersecting, divide the rock 
into masses not larger than a nut, causing it to exhibit a curious reticulated appearance where 
the surface is weathered. A similar character was observed in the serpentine at New Almaden. 
At the Orphan Asylum, the compact portions of the rock, which break with a conchoidal 
fracture, are also traversed by thin seams or bands of a green mineral, non-crystalline, and 
about as hard as serpentine. It is delicately shaded in lines parallel with the walls, looking 
somewhat like a narrow green ribbon, of light color. With the exception of some small nodules, 
of a white powder, probably arragonite, these were the only mincrals observed in connexion 
with this rock. The slaty portions were light-green in color, and had a greasy feel, like talc, 
rendering the surface of the rocks at Fort Point very slippery and difficult to climb. 
The hard and dark-colored portions of the rock were used in the construction of the Orphan 
۱ According to the notes of Lieutenant Belcher, serpentine is found on the north shore of the Golden Gate, between Point 
Bonita and Point Diavolo. —Zoology, &c., of Captain Beechey's Voyage. 
Se Eee i Qu UTI EN A he 
ARTE 
