366 University of California Publications. [Geology
The
writer inclines to the belief that this rock was originally a facies of
the Franciscan radiolarian chert. He has seen distinctly altered
cherts that have a somewhat similar texture and mineral appearance
under the microscope. The Franciscan cherts grade over insensibly into
siliceous iron ores and in a number of localities have associated with
them deposits of manganese dioxide. This would explain the high iron
and low alumina content and the association with manganese stringers.
Much or all of the soda and other oxides in part may have been
introduced during the metamorphism, as in the case of certain
crocidolite schists of the Coast Ranges which the writer has found to
have been derived from ferruginous cherts by a similar process.18
Of
the rocks described as associated with the veins the greenstone
(altered diabase) is the most abundant and the one most commonly in
contact with the veins in moderately altered condition—especially
towards the east end. On approaching the central part of the zone of
veination, however, the alteration increases very greatly, the
original pyroxenic constituents disappear and the chief constituents
are the new-formed amphiboles. The old structures are entirely lost. In
part we may refer to the material as soda-amphibole schist.
A
still further alteration is caused by the leaching out of the
feldspathic constituents, leaving the rock in a more or less porous
condition, as occurs on the left side of the cut shown in plate 32.
This
rather porous rock near the veins may be thoroughly impregnated with
natrolite for a fraction of an inch or several inches from the vein;
also it is in this rock that the spaces occur covered with free-growing
amphibole needles on which the natrolite groups are perched as already
described.
SEQUENCE OF EVENTS. The
field relations and lithologic characteristics indicate that the rocks
in which the benitoite-bearing veins occur are a detached mass of the
Franciscan series, showing both igneous and sedimentary facies, that
was included in the serpentine at the time of its intrusion.
is Louderback and Sharwoocl: Bull. Geol. Soc. Am., 18 (1906), abstract p. 659.
