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MINERAL AND CHEMICAL COMPOSITION. 19
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of
these quartz veins possess very sharp boundaries, but others are only
vaguely delimited from the pegmatite. The quartz veins of the latter
type are particularly likely to contain scattered crystals of
orthoclase-microcline and some muscovite and black tourmaline. Black
tourmaline is also in some of the veins associated with quartz alone,
the two minerals being in many places intimately intergrown. In some
narrow veins the black tourmaline may be more abundant than the quartz.
In one place the genetic relation between a quartz vein and the
pegmatite was shown in the unequivocal manner illustrated in figure 3.
Contemporaneous quartz dikes in pegmatite are also well exposed in the
Boothbay Harbor region. (See p. 166.)
At
a large number of localities, where injection gneisses are associated
with pegmatite, quartz stringers in the gneiss can be traced into
continuity with the pegmatite. A striking instance of this is
illustrated in Plate IV, B.
In
the Maine deposits quartz is very rarely found in distinct bands in
dikes or sills of pegmatite. In a single small dike in Topsham some
concentration of quartz in the central portion of the dike was
observed, the feldspar being concentrated mainly along the walls.
Four
thin sections of rose, white, and gray quartz from the larger quartz
masses in the pegmatites were examined under the microscope. One of
these consisted of a single quartz individual, but the other three
showed several interlocking quartz grains within the small area
covered by the microscope slide. The quartz in all of these showed
little or no strain except along an occasional zone of fracturing and
recrystallization such as that shown in Plate VI. In one specimen of
quartz from a quartz-rich pegmatite near Cumberland Mills (see p. 62)
all of the grains are much strained and are granulated along their
borders. Like the development of mica-coated shear planes in certain
pegmatites, this indicates slight local shearing movements subsequent
to some of the pegmatite crystallization. Such phenomena are the
exception, however, and not the rule.
Fluidal cavities.—Fluidal
cavities of microscopic dimensions are abundant in most of the
pegmatite quartz examined. They are very similar in character in almost
all the quartzes, characteristic forms being shown in figure 4. Nearly
all contain a vacuole or gas bubble, which in the larger cavities
reverses its position in the cavity when the
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