104 E. P. CAREY AND W. J. MILLER 
SLICKENSIDED SERPENTINE 
This aspect of the serpentine in most respects closely corresponds 
to that described by Dr. Palache.t. The rock is varied in character, 
being found from a gray to a light green shade, and is quite soft. 
The plane slickensided faces, often greasy and striated, predominate 
over curved faces, so that in many places it has a foliated or schistose 
structure. This phase occurs in rather extensive zones which appear 
to be fault zones, and also along joint planes in the peridotite-serpen- 
tine, which are frequently planes of movement. In some cases the 
slickensiding has extended out from the joint planes, involving the 
peridotite-serpentine until it remains as a core in the center of the 
slickensided variety. 
While at the Potrero the massive facies is very subordinate in 
amount, in the Los Lagrimas Hills and the Oak Hill area the reverse 
is the case, and the bowlders are represented in a few places in the 
beginning stages of formation. 
MOTTLED SERPENTINE 
Associated with the massive green serpentine in and about the 
quarry occurs a light green facies in which roundish light colored 
areas are found in the green or grayish-green variety making up 
about half the rock, and giving to it a very striking and mottled appear- 
ance. The light spots vary in size from points barely visible to the 
size of a hen’s egg or evenlarger. ‘The microscope shows that the mesh 
structure is present in the least decomposed parts of the white areas 
and is continuous from the contiguous green part, in some cases 
gradually disappearing as the center of the white area is approached. 
Small magnetite crystals are also present, though not so numerous, 
and have a similar distribution. Often small cores of the greenish 
serpentine occur within the white areas, frequently near the center. 
Qualitative analysis demonstrated abundant silica and magne- 
sium, indicating that it is essentially magnesium silicate, and that it is 
derived from the mineral serpentine by secondary changes, possibly 
accompanied by the leaching out of iron. The frequent occurrence 
of this white material in veins, and the fact that the surface of the 
t “The Lherzolite Serpentine and Associated Rocks of the Potrero,” S. F. Bulletin 
of the Department of Geology of the University of California. 
