Vol. 5] Louderback.—Benitoite. 365
bonaceous
matter that followed the lamination. The new feldspar as determined in
several places is also albite. Throughout the rock small needles are
beginning to develop, the more common one observed being actinolite.
Some very small colorless ones also occur.
A
peculiar rock occurs towards the western part of the zone. It is dark
colored, aphanitic, dense and fine banded, as if from the effects of
original lamination. Manganese dioxide stringers are common. Under the
microscope it is seen to be a fine micro-crystalline aggregate,
consisting in part of dark patches made up largely
of a brown mineral in short minute prisms with apparently straight
extinction, negative elongation, high refractive index and moderately
strong double refraction. In the preparations at hand it was not
determinable. In parts veinlets and areas are numerous, carrying
albite, chlorite, glaucophane, etc.
In
the hope that its origin might be indicated by the chemical
composition, Dr. Blasdale undertook the analysis with the following
results.
