CHAPTER XII. 
GEOLOGY OF THE VICINITY OF SAN FRANCISCO. 
ENUMERATION OF THE PRINCIPAL FORMATIONS —GRANITE NORTH AND SOUTH OF THE GOLDEN GATE.—GEOLOGICAL MAP.—SAN FRANCISCO 
SANDSTONE.— POINTS AT WHICH IT IS EXPOSED.—SECTION AT YERBA BUENA.—SANDSTONE AND SHALES.—DECOMPOSITION OF THE ROCK.— 
GLOBULAR MASSES, THE RESULT OF DECOMPOSITION,— COLOR OF THE ROCK.—LITHOLOGICAL CHARACTERS.— REMAINS OF PLANTS.—STRATA 
UNDER THE 0175 RESEMBLANCE TO TRAP ROCK.—POINT LOBOS.—ANGEL ISLAND.—STATE’S PRISON QUARRY.— SECTION OF THE STRATA.— 
DISLOCATION OF A BED OF THE SANDSTONE.—MARIN ISLAND.—BENICIA SANDSTONE.—NAVY POINT.—CONGLOMERATE.—SECTION OF THE 
STRATA AT NAVY POINT.—HARD BLUISH-GREEN MASSES.—PROBABLE SYNCHRONISM OF THE STRATA WITH THOSE NEAR SAN FRANCISCO.— 
EXTENSION OF THE STRATA SOUTHWARDS NEAR MOUNT DIABLO.—SANDSTONE AT NEW ALMADEN, SAN JUAN, AND NORTH OF THE GOLDEN 
TE 
GATE.— BELLINGHAM BAY SANDSTONE PROBABLY THE SAME.—ÀGE OF THE FORMATION.—Fossı ROBABLE TERTIARY AGE.—SECTION 
FROM SAN FRANCISCO TO THE PACIFIC.—METAMORPHIC SANDSTONE.—JASPERY OR PRASOID CHAR E CK3.— GRANITE.— 
TRAP.—SERPENTINE.—FORT POINT.—DIALLAGE OR BRONZITE.— (LOBULAR CHARACTER OF THE ROCK.—STRATA IMBEDDED IN THE SER- 
PENTIN OST TIARY AND ALLUVIAL DEPOSITS.—ENCROACHMENT3 OF THE SEA.—DRIFT OR SURFACE ACCUMULATIONS.—SAND 
DUNES.—BEACH ON THE PACIFIC SIDE —HAPPY VALLEY.—STRATIFICATION AND RIPPLE MARKS,—ARTESIAN WELLS AT SAN FRANCISCO 
AND 8AN JOS 
The principal rock formations of the vicinity of San Francisco are sandstones and shales, 
together with erupted trappean rocks and serpentine—all, probably, of comparatively recent 
geological age. Granite and the associate rocks were not seen near San Francisco, but they, 
probably, form the central portion of the San Francisco or San Bruno range, as far south as 
San José, or the mines of New Almaden, where a white crystalline limestone occurs. Granite 
is found in the Santa Cruz Mountains, and at Point Pinos, Monterey. It also outcrops on 
the coast north of the Golden Gate, forming the projecting headland called Punta de los Reyes, 
and the group of small islands , about twenty miles from the Golden Gate, known as the Far- 
allones. 
A small Geological Map of the vicinity of the entrance to the Bay of San Francisco is pre- 
sented with this chapter, and will serve to show the geology of the headlands and the adjoining 
shores.) A fine-grained, compact sandstone, associated with shales, is the prevailing rock. It 
underlies the city of San Francisco, and is exposed along the shores of the bay, both north and 
south of the city, forming the principal promontories and points, and several islands. On 
entering the bay, from the Pacific, the rock is first seen at Point Lobos, the outer point, and 
again at North and Tonquin points. It borders a part, at least, of the Golden Gate, on the 
north, and forms the shores of Richardson’s and Saucelito bays. Angel and Yerba Buena islands 
are also of this sandstone formation. In several places, hills and ridges, of over two or three 
hundred feet in elevation, are formed entirely of this rock, and the wearing action of the sea, at 
their base, and the break in the ranges forming the Golden Gate, have produced favorable sec- 
tions where the characters of the strata may be studied. Availing myself, therefore, of these 
* Since the printing of this report commenced, I have had the opportunity of seeing the volume on the Zoology of Captain 
Beechey’ s Voyage, in which I find some observations on the geology of the vicinity of San Francisco, prepared from the 
notes and collections of Lieutenant Belcher, by Professor Buckland. A map of the headlands, embracing about the same 
area as in the map accompanying this report, is also given in illustration. This is colored around the shores so as to indicate 
the several formations. Serpentine, sandstone, and jasper rock are represented.—Zoology of Captain Beechey's Voyage to the 
Pacific and Behring’ s straits in 1825,~’26,-’27, and 28. 4to, London, 1839. : 
19 F 
