338 University of California Publications. [Geology
and
the planes of schistosity lie roughly parallel to the outcrop. This is
especially noticeable at the east end of the zone -where the rock is a
fine-grained greenstone, and along the line of the zone shows incipient
schistosity and is traversed by narrow veins of natrolite lying chiefly
in the planes of foliation.
The
cracks and spaces in which the veins have been deposited are not always
completely filled, and drusy cracks and geodal cavities are quite
common. As might be expected, many of the best crystals, and almost all
of the better specimens, are obtained from such drusy spaces. Often
the filling has proceeded so far that while a number of the crystals
from opposite walls have united, the majority are still free. These may
be split open. Plate 29 shows a slab three feet long and eighteen
inches maximum width which has been so opened, exhibiting the
corresponding drusy surfaces. Plate 30 and plate 33 show smaller
surfaces more in detail.
Sheeting
is very marked in the face of the open cut, as can be distinctly seen
in plate 31. To the right of the rope it is very well developed with
vertical planes. It occurs in other parts of the mine also. Crushing is
evident here and in other places where sheeting is not developed.
It
is interesting in this connection to note that eastward beyond the
limits of the vein-bearing rock-lens, and in continuation of the
direction of the zone of mineralization, the serpentine is badly
brecciated.
Evidences
of movement are very plentiful—both fault-planes and displacements. In
the face of the open cut (plate 31) the rope lies on a very distinct
plane of movement. To its right is crushed, sheeted, and altered
greenstone; to the left the veinstone and tough impregnated wall-rock
is seen broken into separate blocks.
This
crushing and faulting of the mineralized zones allows the ready seepage
of water, and therefore favors decomposition. The rock outside the
impregnation zone has suffered the most from this, and especially in
the sheeted areas has suffered so from oxidation and other changes that
it is often impossible to get a definite idea of its original nature.
In the mineral druses. wThere permeable, the waters have deposited a layer of limonite
