368 University of California Publications. [Geology
further
interchange or passage of material from or to the rocks. The setting up
of active deposition, the stopping of metamorpho-genic diffusion into
the wall-rock, and the substitution of natrolite for albite as the
precipitated molecule, all point to a marked lowering of temperature
both of ascending solutions and of the country rock.
The
benitoite and neptunite do not appear anywhere to follow the natrolite
into the wall-rock. In a few places the neptunite appears to be
partially surrounded at its base with amphibole-natrolite matrix but it
is practically limited to the confines of the vein proper. Both
benitoite and neptunite are attached directly to the wall so that they
often commence to deposit before the wall received any coating of
natrolite. They also occur in the central portion of the vein entirely
surrounded by natrolite, showing that at these points the latter
commenced depositing first. They also occur in the natrolite druses and
project freely from the surface, showing that they were among the last
materials to separate. We conclude therefore that benitoite, neptunite,
and natrolite were deposited contemporaneously throughout the period of
vein filling.
The
order of crystallizing power—the crystalloblastic order— arranged in
decreasing magnitude is neptunite, benitoite, natrolite. Natrolite is
found in abundance surrounding neptunite and benitoite, molding itself
to suit their form and showing the smooth surfaces of their imprint
when separated. A few cases were observed where benitoite crystals are pierced by neptunite prisms or have partially grown around them.
The
next events recognized are those of descending solutions,
decomposition, etching, limonite coatings, etc., as already referred to
elsewhere.
DISTRIBUTION OF MINERALS IN VEINS. The
workings are so shallow at the mine that there are no data at hand to
indicate the distribution of the various minerals with depth. But it is
of some interest to note that the distribution is not uniform along the
surface. The benitoite is most abundant along the east central portion
of the zone of veination. Passing eastward the titano-silicates
decrease in abundance until the
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