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CONTACT-METAMORPHIC EFFECTS. 33
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the
cavities. Some filling must have been present from which such crystals
derived the materials for their growth. It is probable, therefore, that
immediately after the crystallization of the main body of pegmatite the
miarolitic cavities were completely filled with a gaseous solution,
which may later have liquified and has since disappeared. Water
carrying numerous other substances in solution probably formed the bulk
of this cavity filling. The abundance of quartz crystals on the walls
of these cavities indicates that silica was one of the most abundant of
the dissolved substances.
If
the crystallization of the rock with pegmatitic rather than granitic
texture is due to the presence of larger amounts of gaseous
constituents, greater size or abundance of microscopic fluidal or
gaseous cavities might reasonably be expected in the pegmatite minerals
than in those of the normal granites. With this idea in mind the writer
attempted a microscopic measurement of these inclusions in pegmatites
and associated granites from Maine. On account of the uneven
distribution of the inclusions in bands traversing the minerals
accurate estimates were found to be impracticable and the results were
negative or inconclusive. It was found, moreover, that some of the
bands of fluidal cavities in the quartz of pegmatite were formed later
than shearing movements which had affected the quartz. (See Pl. VI.)
The inclusions in the pegmatite were similar in character to those in
the normal granites of the State and any differences in their size and
abundance in the two types of rocks was not sufficient to be noted on
casual inspection.
Contact-metamorphic effects.—If
the pegmatite magmas are characterized by considerably larger
proportions of gaseous constituents than are present in the granite
magmas and hence by notably greater fluidity, notable differences might
be expected in the contact-meta-morphic effects produced by the two
types of rocks, since such effects are believed to be produced largely
by gaseous and fluid emanations from the cooling igneous masses. Field
observations in Maine fail to show that contact-metamorphic effects due
to the intrusions of pegmatite are notably greater than those produced
by the granites. The effects produced by both are usually slight and in
many instances almost nil. In many places masses both of pegmatite and
granite cut across the foliation of schists without any distortion of
the latter, the contacts being of knife-edge sharpness. In other
places, however, pegmatite has produced a notable softening of the
bordering rock, though this effect is usually apparent only close to
the contact.
A striking instance of this effect was observed about 2\ miles
northeast of Paris village, where a pegmatite mass 2 to 3 feet across
and several smaller masses are intrusive in schists of probable
metamorphic-sedimentary origin. (See Pl. X, A.) Although the
63096°—Bull. 445—11-----3
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