CRYSTALLINE ROCKS OF OAK HILL AREA 161 
miles along the Los Lagrimas Hills southeast to the Mount Hamilton 
Range. It is about eleven miles from the serpentine of the New 
Almaden region described by Becker. 
In most respects it is similar to the serpentine of the San Fran- 
cisco Peninsula, Angel Island, Crystal Springs Lake, and numerous 
other occurrences in the Coast Ranges; and it is probably identical 
in age with them. It is associated with radiolarian cherts and sand- 
stones which belong to the formation described by Professor Lawson? 
as San Franciscan, and by Dr. H. W. Fairbanks? as Golden Gate. 
The serpentine mass here represents an intrusion in the nature of a 
boss or very large dike. ‘The outcrop reaches a width of one and a 
half miles. The extent below the surface cannot be determined 
because in most places it is buried beneath the alluvium which covers 
the valley floor. 
The prevalence of schist about the periphery of the serpentine, 
together with the fact that the jaspers, where they occur, are fre- 
quently contorted and otherwise altered, would suggest that serpen- 
tine was capable of affecting to a considerable degree the rocks into 
which it was intruded; yet this evidence is not conclusive. 
Petrographic characters.—The serpentine varies greatly in char- 
acter. It typically consists of crystals of diallage imbedded in a 
groundmass varying in color from a light to a dark green, consisting 
chiefly of the mineral serpentine, in which are numerous grains of 
magnetite. Enstatite and iron-pyrites are found very sparingly, 
while a large portion of the slides show residual grains of olivine. 
The process of serpentinization is also observed in some of the imbed- 
ded diallage crystals working in along the cleavage planes from the 
outside to the center, although this mineral is usually fresh. 
This massive phase of the peridotite-serpentine varies in composi- 
tion from a rock, in which the glistening face of a diallage crystal may 
be seen here and there imbedded in serpentine, to one made up essen- 
t Geology of the Quicksilver Deposits of the Pacific Slope, Monograph XIII, U. S. 
Geological Survey. 
2 Geology of the San Francisco Peninsula, Fifteenth Annual Report, U.S. Geological 
Survey. 
3 “The Stratigraphy of the California Coast Ranges,” Journal of Geology, Vol. III 
(1895), PP- 415-33- 
