CRYSTALLINE ROCKS OF OAK HILL AREA 153 
this area is a quarry from which the city of San José obtained road 
material for a number of years. ‘The road material consisted largely 
of diorite intruded into partially serpentinized peridotites and pyroxen- 
ites, with which they are very intimately associated. 
The area mapped and studied in detail comprises nearly three 
square miles, namely the group of hills south of the Oak Hill Ceme- 
tery and west of the Monterey Road. This group of hills rising 
out of the Santa Clara Valley will be designated in this paper as the 
Oak Hill area, while the term “Quarry Hill” will be applied to the 
hill in which the quarry is situated.t The prevailing rock of the 
region is serpentine, which shows unmistakably its origin from 
peridotite. The transitions have been most carefully studied in and 
near the quarry. Not less interesting, however, are the pyroxenite 
and gabbro occurrences which are intimately associated, the pyroxen- 
ite showing every step in the transition into serpentine, on the one 
hand, and apparently into the different types of gabbro, on the other. 
Several diorite dikes occur cutting through the more basic rocks, and 
glaucophane schist has been found in a number of excellent exposures. 
The region is further remarkable for the extent and variety of 
intrusives of more acid rocks, gabbroid and dioritic in character. 
Sandstones and radiolarian jaspers make up a small part of the 
area and are the only rocks of sedimentary origin. 
TOPOGRAPHY 
The Oak Hill area comprises a group of well-rounded crests, 
part of an old erosion plain rising out of more recent valley alluvium 
which surrounds the hills on all sides. These hills really form an 
integral part of the Mount Hamilton Range, and are practically 
connected with this range by a low line of peridotite-serpentine hills, 
sometimes known as the Los Lagrimas Hills, which outcrop to the 
southeast of the Monterey Road. 
Of the nine fairly well-defined crests of the area, the highest 
is 438.5 feet above the level of the valley. The trend of the crests 
is approximately northwest and southeast, which corresponds to the 
general strike of the Coast Ranges in this region. 
t Work on the rocks of the Oak Hill area was carried on during the two years 1901 
and 1902. The petrographic investigations were carried on at Stanford University 
under the direction of Dr. J. P. Smith. 
