62 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
meter.
It is composed of about two-fifths quartz, two-fifths feldspar, and
one-fifth hornblende, with subordinate titanite and biotite. Some of
the hornblende crystals are 1.2 millimeters in length. Their tendency
to parallel elongation and to greater abundance in some layers than in
others gives the rock its schistose character. Biotite is also most
abundant in the layers that are most hornblendic. The feldspar is
principally orthoclase with a little microcline and plagio-clase near
andesine. Many of the quartz grains show strain shadows, but there is
no other evidence of dynamic action.
Alternating
with this rock are bands of very dark gray to nearly black
hornblende-biotite schist with lustrous cleavage faces. An intermediate
phase is a dark-gray hornblende schist with a few narrow quartz bands
up to about one-eighth inch across.
Under
the microscope this rock is seen to consist of quartz, plagio-clase,
and hornblende. The plagioclase is andesine and is about equal to
hornblende in abundance. Quartz is slightly less abundant than either.
Titanite is subordinate. Occasional narrow bands are more coarsely
crystalline and are largely quartz, with some feldspar. Their grains
interlock intimately with those of the finer portions of the rock. The
schistosity, as in the more acidic bands', is due to the concentration
of hornblende along certain planes and of quartz along certain others
and to parallel elongation of many of the hornblende crystals.
If
these schists represent original sediments their recrystallization has
been so complete as to obliterate all traces of such an origin. The
abundance of feldspar, on the other hand, especially in the more basic
bands, renders it much more probable that they are primary or flow
schists.
The
pegmatite in some cases is in sharp contact with the gneiss, and the
contacts may parallel or cut across the foliation. In other cases the
pegmatite seems to grade into the gneiss with such completeness as to
indicate either that portions of the gneiss were not yet completely
solidified when the pegmatite was intruded or that the pegmatite
produced locally very complete recrystallization in the schist. The
pegmatite is a typical biotite pegmatite showing much graphic granite
and a few crystals of pure feldspar 4 or 5 inches across.
WESTBROOK.
A
quartz deposit which was worked to a small extent many years ago is
located about 1 mile northwest of the village of Cumberland Mills. The
quartz forms part of a pegmatite dike intruding mica schist and
granodiorite. The width of the dike varies from 2 to 10 feet, and its
trend is nearly north and south. Most of the mass is typical
granite-pegmatite of moderate coarseness, but with this is associated a
body of nearly pure white quartz, which in places
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