I'M I Hi
G E O L O G Y . 175
The Island of Los Angelos is of very confused formation. Its eastern side is sandstone,
with occasional jasper rooks; its western side exhibits sand-stone, conglomerate,
clay-slate, and serpentine; its south side, bluish earth, (apparently decomposed serpentine),
and jasper beds containing red siliceous nodules, and much iron pyrites. The
superstratum of this island is almost entirely composed of the debris of sand-stone and
jasper rocks, a little slate and bluish earth, and betrays appearances of violence. I t is
about 900 feet above the level of the sea.—B.
The cliffs of the main land opposite the north-west shore of the Island of Los
Angelos afford masses of actynolite and beds of mica slate and talc slate.
The Island of Molate, about four miles north of Los Angelos, appears at a distance
to be of a red colour, and contains much red jasper, and in a small portion of the cliff
black ferruginous slate.—C.
In the Island of Yerba Buena, the perpendicular cliffs west of the bay are formed
of clay slate at their base, whilst the superincumbent rock is sand-stone, for the most
part in angular masses, and without distinct stratification. The clay-slate is much contorted,
arched, and wavy, assuming an east and west direction, and dipping chiefly to
the south at a considerable angle. The sand-stone shows itself in the point that forms
the eastern part of tbe bay.
The rounded hills of tbe peninsula on which the Presidio of San Francisco is
placed, are variously formed of sand-stone, loose sand, serpentine, flinty slate, and jasper.
The westernmost hill, which rises from tbe sea between the fort and the Punta di los
Lobos, is serpentine. The north declivity, on which tbe quadrangle of the Presidio is
built, is sand-stone. To the eastward of this the serpentine again forms a hill of equal
if not greater height. Tbe hill to the westward of the Mission is serpentine. Th a t which
rises to the south of it exposes a bare and scarped brow of flinty slate and jasper. Rocks
of a similar nature protrude through the surface of the soil of the hills which separate
San Francisco from the extensive valley of Santa Clara (Las Salinas), about six leagues
to the southward. These bills are called Sierras di los Saraburnos, and terminate on
the north in a rocky prominence, in the harbour east of the inlet of the Mission.
The range of mountains, Las Sierras del Sur, which bound the above valley to the
south, expose flinty slate approaching to jasper, a little north-west of Las Pulgas, and
about eighteen miles east-south-east of the Mission of San Francisco. Between the
Missions of Santa Clara and Santa Cruz, these mountains form four parallel ranges, the
two middle ones highest (about 1500 feet), with steep declivities: the two first valleys
are narrow; the third is more extensive, leading to the fourth range, which is considerably
lower than the others. Tbe first two ridges are composed of serpentine and a
jaspery rock, the third principally of sand-stone and occasionally jasper, aod the fourth,
that nearest Santa Cruz, entirely of sand-stone, the upper part being mostly decomposed
into loose sand. Petrified bones of a cylindrical form were found in this cliff of sand or
loose sand-stone in 1827.
Where this range approaches the road from Santa Clara to San Juaii, nearly halfway,
the northern declivity is covered with fragments of serpentine, aud a little farther
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