It
is significant that numerous dikes of granite also exposed along this
shore never exhibit such irregular swelling and thinning, but are
nearly parallel-walled even where intruded parallel to the foliation of
the bordering schists.
On
the point due north of Cabbage Island the rocks are almost entirely
granite and pegmatite associated in a very irregular manner. The
pegmatite forms dikes of varying width and irregular boundaries in the
granite and also forms narrow stringers and wholly irregular patches.
In general the change from one rock to the other is rather abrupt,
although characterized by complete crystallographic continuity. In the
places where the association is most intimate and irregular it is
difficult to see how the granite could have been wholly solidified at
the time of the pegmatite crystallization.
The
granite is gray to pinkish, with a faint local foliation. The average
size of grains is about one-half to three-fourths of a millimeter. The
texture, is typically granitic with quartz > orthoclase and
micro-cline > oligoclase > biotite > muscovite. The quartz
shows undulatory extinctions. Some of the smaller quartz crystals are
inclosed by orthoclase or oligoclase and show rounded outlines. Some of
the quartz also crystallized earlier than or contemporaneously with the
biotite crystallization. The bulk of the quartz, characterized by more
irregular outlines and larger grains, is a later crystallization than
the biotite and appears to be about contemporaneous with the feldspars.
Among the feldspars orthoclase is present in greater abundance and
larger grains than microcline. Oligoclase is almost equal to the potash
feldspar in abundance. Many of the feldspar crystals inclose small
crystals of muscovite, which are apparently original. Some micrographic
intergrowths of feldspar and quartz occur.
The
pegmatite is characterized by the same minerals as the granite. Quartz
is the dominant constituent, with orthoclase and microcline second and
oligoclase third. Biotite dominates over muscovite, but is less
abundant than in the granite. The quartz exhibits little or no
undulatory extinction. Some of the grains exhibit crystal outlines on
certain sides, but the outlines of others are very irregular. The
feldspars exhibit only slight decomposition.
It is notable that both rocks carry the same minerals in the same order of abundance.
In
general the pegmatite characteristic of the Boothbay Harbor region
shows considerable uniformity in mineralogical make-up.
Characteristically it shows irregular crystals of
orthoclase-microcline, ranging in diameter up to 6 inches, surrounded
by a less coarsely crystalline association of potash feldspar, white to
gray or amber-colored quartz in masses sometimes several inches across,
small amounts of nearlv white plagioclase, and varying proportions of
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