26 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
cutting
across them. Many small garnets with well-developed crystal forms are
partly or wholly inclosed by muscovite. Lath-shaped crystals of biotite
bordered by muscovite are sometimes found, the cleavage planes of the
two micas being absolutely coincident. Crystals of tourmaline lying in
somewhat flattened form between the plates of a mica crystal are
common, and some small, colored specimens are of much delicacy and
beauty. At Black Mountain, in Rumford, spodumene was observed
intimately inter-grown with quartz.
Mica.—In
many of the pegmatite bodies muscovite is not evenly distributed
throughout the mass but is most abundant in certain zones. (See PI. IX,
A.) These zones appear to be distributed through the pegmatite
in a totally haphazard manner, bearing no relation to the general form
of the pegmatite mass nor to the position of the wall rocks. The
central portions of these muscovite belts for a width of a few inches
consist of an aggregate of heterogeneously arranged muscovite plates,
few of them more than one-fourth inch in diameter. (The hammer head in
the illustration, PL IX, A, rests on one of these central
fine-grained portions.) They are commonly plane or only gently
undulating throughout their length and, being lines of weakness, are
usually marked by a fracture plane. From this fine-grained portion
spearhead-shaped books of muscovite, showing wedge structure (see p.
139), project in a direction nearly at right angles to the general
plane of the mica belt; some of these muscovite books are a foot in
length. This peculiar distribution of muscovite is not readily
explainable; but it seems to represent a muscovite crystallization
proceeding not from a single center, but from a plane or from a large
number of centers lying in nearly the same plane. In the pegmatite of
the Black Mountain mica mine in Rumford, -where the mica locally
constitutes three-fourths of the pegmatite mass, the elongate mica
books near the schist wall rock tend to orient themselves with their
long axes perpendicular to the contact. In a number of quarries
muscovite crystallization about a center is exemplified by the presence
of nearly equidimensional aggregates, some of them 5 feet across,
consisting of small heterogeneously arranged plates averaging about
one-fourth inch in diameter. From their peripheries these muscovite
aggregates send off spearhead-shaped muscovite books into the
surrounding pegmatite.
Biotite
may occur in isolated lath-shaped crystals penetrating the pegmatite in
all directions or in radiating aggregates of such crystals.
Gem-bearing pegmatites.—Pegmatites
particularly rich in gem minerals exhibit peculiarities of structure
not present in the normal rock. Lithium minerals, such as colored
tourmalines, lepidolite, spodumene, etc., and the soda feldspar
clevelandite, are concentrated in a zone which usually parallels the
plane of greatest dimension of the
