364 University of California Publications. [Geology
COUNTRY IN WHICH THE VEINS ARE FORMED.
As
already stated, the chief rock of the surrounding country is
serpentine. This is of a type common in the Coast ranges and in general
derived from the alteration of a peridotite. Small areas of a pyroxenic
facies occur. Nowhere so far as known do the veins under discussion
occur in actual contact with the serpentine, although it surrounds the
deposit and is frequently not many yards distant from them.
The
rocks immediately associated with the veins are all more or less
altered, and this alteration is greatest close up to the zone of
veination. In the less altered parts both igneous and sedimentary
types are recognized. The more common type has in the field the usual
appearance of the Franciscan greenstones. Under the microscope it is
seen to have originally possessed a diabasic structure. In some
specimens the augite is still largely intact. The feldspars however are
recrystallized into a fine granular mass. Yet they often show very
clearly by the outline of the granular areas the lath-shaped forms of
the original feldspars and the relationship to the augites that
characterize the diabase structure. Some titanite is present. In a
somewhat altered specimen the augite is more or less altered into
chlorite, while in the feldspathic layers small greenish or bluish
needles are commencing to form in some cases actinolite, occasionally
glaucophane, or some other geologically related amphibole. The new
feldspar is at least in large part albite.
On
the south hillslope below the east end of the deposit is a spheroidal
gabbro. The grains and prisms of monoclinic pyroxene are in part
altered to chlorite. The labradorite is more or less decomposed and
otherwise altered and the rock is impregnated with calcite. It does
not come in contact with the veins at any point.
Other
rocks are found having the characteristics commonly displayed by the
more altered Franciscan sandstones or grey-wackes. Under the microscope
the light colored constituents which make up the bulk of the rock are
seen to be entirely recrystallized into very fine granular aggregates.
The original structure is preserved by the dark films of ferruginous or
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