146 
GEOLOGY. 
natural exposures, and of the excavations made in quarrying, the strata were examined at the 
following places: City of San Francisco, Points Lohos, San Jose, Tonquin, and North, Terha 
Buena island, Alcatrazes island, Angel island, Point San Quentin, north side of the Golden 
Gate, and along the shores of the Sancelito hay. Bocks, probably identical in age, were also 
examined at Benicia, New Almaden, and between San Juan and Monterey. 
One of the best sections, where the lithological characters of a part of the formation are fully 
exposed, is at Yerha Buena, the island directly opposite San Francisco. This is composed 
entirely of the sandstone and shale; the strata are laid hare by the action of 
the water around the base of the island, and form a hold rocky shore, which 
in many places appears to offer great resistance to the persistent denuding 
action of the waves and strong currents. On approaching the island from 
the west the evidences of stratification become visible, and the beds are seen to dip westward, 
toward the observer. There are also several places where the strata are bent and contorted, 
as in the figure. 
On the south end of the island a quarry has been opened. At this point the edges of the 
strata are distinctly exposed, and are seen to dip about 20 N. of E., at an angle of 45°. The 
position and general characters of the strata may he best exhibited by a short local section, the 
total thickness represented being about two hundred feet. The thicknesses of the compact beds 
SECTION OE SANDSTONE AND SHALE, YERBA BUENA. 
of sandstone vary from a few inches to six and eight feet; the layers alternate with beds of 
argillaceous slates and shales. All the weathered surfaces of this series of beds are of a rusty- 
brown or drab color, which extends throughout the rock to a depth of from ten to twenty feet, 
down to the limit of atmospheric influences. There are, however, parts of the upper beds that 
have not yet been reached and changed by decomposition; these parts are found in the condition 
of spherical or ellipsoidal masses, from which the weathered parts scale off in successive crusts. 
These nuclei have the appearance of great rounded boulders, and have accumulated in great 
numbers at the base of the cliff. They are of various sizes, hut are smallest in the upper parts 
of the strata, near to the surface. 
This spherical or globular condition does not appear to he the result of any peculiar arrange¬ 
ment of the material of the strata, a concretionary action, such as takes place 
in the igneous rocks, hut is probably due to decomposition—the result of the 
absorption of infiltrating waters, charged with impurities. A solid and homo¬ 
geneous cube of sandstone thus exposed, under conditions favorable for absorp¬ 
tion of the water on all its sides, would decompose most rapidly on the angles, 
producing a succession of curved surfaces gradually approaching a sphere, as 
is represented in the figure. 
These necessary conditions for the infiltration of water exist in the strata. Each layer of rock 
is divided into blocks of various sizes by joints, or cleavage planes, similar to those traversing 
