GEOLOGY. 
175 
The Island of Los Angelos is of very confused formation. Its eastern side is sand- 
stone, with occasional jasper rocks ; its western side exhibits sand-stone, conglomerate, 
clay-slate, and serpentine ; its south side, bluish earth, (apparently decomposed serpen- 
tine), 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. It 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 A^erba 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 con- 
torted, 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 the bay. 
The rounded hills of the 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 the sea between the fort and the Punta di los 
Lobos, is serpentine. The north declivity, on which the 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. The hill to the westward of the Mission is serpentine. That 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 hills are called Sierras di los Samburnos, 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 conside- 
rably lower than the others. The first two ridges are composed of serpentine and a 
jaspery rock, the third principally of sand-stone and occasionally jasper, and the fourth, 
that nearest Santa Cruz, enth'ely 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 Juan, nearly half- 
way, the northern declivity is covered with fragments of serpentine, and a little farther 
2 a 2 
