36 
traces of augite, olivine or other original iron-magnesian sili- 
cates. 
Western Area. The porphyrite of this area varies from 
light greenish to dull red and purplish in color and is well de- 
scribed by the term /e/sz/ze. So far ав observed none of its 
mineral constituents are of such dimensions as to be recog- 
nizable by the unaided eye, nor is it at all amygdaloidal, a 
feature so pronounced in the melaphyrs. The purple variety, 
in the thin section, shows a densely microlitic base, with much 
opacite, carrying numerous porphyritie feldspars, which are 
mostly, if not all, triclinic. A few of the feldspars show no 
twinning strie; but I eannot obtain such measurements as 
would prove these to be certainly monoclinic. 
One of the sections examined shows a single corroded bleb 
of original quartz, carrying fluidal cavities and moving bubbles. 
Large dusky apatites are not rare. In the greenish variety 
there is also present alittle viridite from the feldspars, and a 
few granules of secondary epidote. The porphyrite presents, 
usually, a well-marked fluidal structure. Subjected to chemical 
analysis, the green variety yielded 58.02 per cent. of silica, 
and had a specific gravity of 2.73. The red variety is more 
felsitic and also more decomposed. Two samples, light red, 
and dark red in color, yielded silica as follows : light, 56.66 per 
cent. ; dark, 56.25 per cent. 
Black Rock.—Some of the black rock porphyrite more re- 
sembles the melaphyrs in external appearance than does that 
of the western area. Macroscopically it is a very compact and 
hard, greenish gray to dark purplish and nearly black rock, 
bearing small whitish feldspars, and secondary epidote in crys- 
tals and granular aggregates of sufficient size to be distinguish- 
able by the naked eye. 
Thin sections, under the microscope, show it to consist of a 
dense ground-mass of needle-like and short, stout feldspar micro- 
lites, interspersed with numerous brilliantly polarizing epidote 
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