160 
GEOLOGY. 
lower end of the hay, there are broad, alluvial meadows, or marshes, and an extensive tract is 
left hare by the tides. The extension of this alluvian inland, at a higher level, forms the broad 
and fertile plains of San Jose. These deposits are bordered by the more dry and sloping plains 
of coarser soil, formed by the debris and wash from the hills. 
On the hills about San Francisco there is a slight formation of drift, either alluvial or com¬ 
posed only of the wash from the hills. It does not show upon the surface in the form of either 
transported boulders or gravel, but is limited in extent, and occupies the lower parts of the 
principal depressions. In excavating a tunnel from the small lake on the west slope of the ser¬ 
pentine ridge, a portion of this formation was cut through, and at its junction with the serpen¬ 
tine rock, about 80 feet below the surface, fragments of wood were taken out. They were im¬ 
bedded in black clay, like sea mud, and were much compressed and flattened out, and partially 
converted to lignite. Bones were also found in this clay, 18 feet below the surface. One of 
them appeared to be the rib of an animal not larger than a deer. A fragment of a large bone 
was also taken from an excavation at the foot of Telegraph Hill. It is about eight inches long 
and four in diameter, and is nearly triangular in its cross-section. 1 
In boring through the earth outside of the old water-line of the city, at the site of the new 
custom-house, in order to ascertain the nature of the foundation, several beds of sand, clay, and 
gravel were found to succeed in regular order for a depth of 60 or 80 feet. This locality is at, 
or below, tide level, and we thus find that the sandstone strata in the channel are overlaid by 
drift or detrital deposits of considerable depth. It is between these accumulations of drift, or 
alluvium, and the rocks, that sheets of water or water-bearing strata are found, and are reached 
by Artesian borings in various parts of the city. 
The sandstone and shales of the hills upon which the city is built are overlaid in many places 
by a covering of soil, which appears to have been derived from the decomposition of the strata. 
This soil is found to be a good material for making bricks, and it is extensively used in that 
manufacture. This fact shows that the rock contains a large per centage of alumina, and the 
presence of oxide of iron is not only shown by the rusted color of the weathered rock, but by 
the deep red color of the burned bricks. Wherever this soil has been cultivated it has been 
found to yield good returns. 
SAND-DUNES. 
On the Pacific side of the San Francisco peninsula there is an extensive sand-beach, reaching 
for several miles north and south, and a long distance inland. A wide area is thus covered by 
loose, dry, sea sand, and it has the aspect and character of a desert. This sand is moved about 
by the wind and is constantly progressing inland, being thrown into wave-like hills, which 
move forward and bury shrubs and trees that lie in their path. The extensive formation of 
blown sand within the city limits has undoubtedly been accumulated by the action of the sea 
winds upon the sand of the beach, it having been transported from one side of the peninsula to 
the other. 
The sand-hills formerly occupied a wide area in Happy Valley, rising in constant succession, 
one beyond the other, to various heights, from 20 to 60 feet or more, rendering the region 
almost impassable for vehicles. Most of these hills are now levelled, aud houses have taken 
their place. Remnants of them may, however, be found along the newly opened streets, where 
favorable opportunities are presented for the examination of their internal structure. Many 
' These bones are in the possesion of Dr. C. F. Winslow, formerly of San Francisco. 
