Ch. 2: Maine Pegmatites: Local Descriptions

Ch. 2: Maine Pegmatites: Local Descriptions Page of 170 Ch. 2: Maine Pegmatites: Local Descriptions Text size:minus plus Restore normal size   Mail page  Print this page
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
Ch. 2: Maine Pegmatites: Local Descriptions Page of 170 Ch. 2: Maine Pegmatites: Local Descriptions
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