340 University of California Publications. [Geology
The
general trend of the outcrop is north 64° west. In the cut the western
part dips about 68° north, while in the face it dips 65° north. At the
east end of the mineralized zone the greenstone with incipient
schistosity and the barren veins of natrolite lying in these planes
strike north 59° west and dip 75° north. In other words, the zone turns
slightly to the south before dying out.
GENERAL RELATIONS OF MINERALS IN VEINS.
The
most abundant mineral of the veins is natrolite, which occurs chiefly
in granular aggregates. Indications of crystal form are largely limited
to the drusy cavities, and even there the natrolite generally forms in
peculiar groups, projecting in small roof-shaped ridges or coxcomb-like
forms, and only very rarely developing the prismatic forms usually
characteristic of natrolite. Some of the druses are filled with very
small needles of green or blue-green amphibole, and lying in the midst
of the cavity supported by these needles the natrolite often occurs as
equant4 polyhedral aggregates of from 1 to 3 millimeters in
diameter, not at all suggestive of the mineral natrolite. Most
conspicuous and beautiful in this white ground of the natrolite gangue
are the scattered idiomorphic crystals of the blue equant or somewhat
tabular benitoite and the brilliant black neptunite prisms, showing
here and there a touch of deep red. These minerals are the
characteristic and more abundant minerals of the benitoite-bearing
veins.
In
plate 30 it is apparent that surrounding the drusy cavity is a layer of
white (natrolite) and that it is followed by a layer of darker color.
This outer layer is of variable thickness—from a fraction of an inch up
to several inches —and is usually present between the white vein
material and the more definitely recognizable wall-rock. It has a
bluish or greenish tint, and looked at closely is seen to show a
granular structure with luster and cleavage much like the vein-stuff.
It is indeed natrolite which is loaded with numerous microscopic
*
Used in the sense of equidimensional or nearly so, in contrast to
tabular or prismatic, as suggested by Cross, Iddings, Pirrson and
Washington, Journ. Geol., XIV (Dec, 1906), p. 698.
