150 
GEOLOGY, 
miles northeast of the State’s Prison quarry, and nearly opposite the Mission of San Rafael. 
The strata are highly inclined, and dip W. 30° N., at an angle of about 60°, trend N. 30° E. 
They are composed of thickly bedded sandstones, alternating with shales, and the stone is 
more like that from Yerba Buena than that taken from the State’s Prison quarry. It has a dark 
bluish-green color and a fine grain, and the black spots are not so abundant as at the other 
localities. Abundance of good building-stone can he obtained at this place, and it is accessible 
for tonnage vessels. 
All the points and headlands around Marin island appear to be formed of similar rocks. 
They outcrop also on the opposite side of the channel, and form low ridges, trending about 
northwest and southeast. In sailing up the bay towards Benicia, the same strata appear to be 
continuous, and seem to be the only formation. The strata are exposed at many points along 
the shores, always with the same rusty drab or browp color. 
At Benicia, the hills are soil-covered, and there are few outcrops; but there is an extensive 
and favorable natural section of the strata at Navy Point, produced by the Straits of Carquinez 
connecting the Bay of San Pablo with Susuin Bay. The current flows nearly at right angles 
to the trend of the outcropping beds ; and their ends are well exposed along the shore, forming 
a bluff of slight elevation. The strata are uplifted, being inclined at an angle of from twenty 
to sixty degrees, and dipping towards the southwest. The trend of the outcrops is 75° west of 
north, and the strata underlie, or rather form, the hill upon which the government buildings 
are erected. 
These strata, as here exposed, differ somewhat from the outcrops at San Francisco, Yerba 
Buena, and other places described; but there is much reason to consider them as a portion of 
the same formation, although they may prove to be much higher in the series. The surface 
decomposition appears to have been more complete, or perhaps the strata were never so hard 
and firm as those nearer San Francisco. 
The great ridge of conglomerate, already described in the notes, forms the prominent feature 
of the section exposed at this point. It is the hardest and most unyielding of all the strata, 
and its resistance to abrasion and atmospheric influences has determined the form of the hill and 
the shape of Navy Point. It forms a prominent object at several points along the surface of the 
ground, and is almost the only rock that appears above the soil in that vicinity. The bed is 
about twenty-five feet thick, and is composed of pebbles and gravel, very round, much water- 
worn, and chiefly derived from the wear of volcanic or eruptive rocks. Their colors are gen¬ 
erally dark; and porphyries, agates, and carnelians are abundant. Their average diameter 
does not exceed an inch, and many are about the size of beans and peas. They are closely 
united by a small portion of finer materials. The strata on both sides of the conglomerate con¬ 
sist of alternate beds of soft and friable argillaceous shales, with an occasional layer of gravel 
and pebbles. Their lithological characters and succession are shown in the following section, 
enumerating the strata from west to east, or in the order of superposition from above down¬ 
wards. The measurements which are given are merely approximate, having been determined 
by the eye, or by pacing in front of the exposed edges in the bluff. 
