48 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
in
the quartzes of both rocks and are of about the same size. The majority
are under 0.005 millimeter, but a few are over 0.01 millimeter in
greatest dimension. They are not notably more abundant in the pegmatite
than in the granite gneiss.
In
a few places the pegmatite is rather sharply delimited from the granite
gneiss in dikelike masses, but for the most part it occurs in the
granite in lens-shaped or roughly spheroidal masses from a few inches
to a foot or more across, coarsest in the center and grading verv
gradually with increasing fineness into the surrounding granite gneiss.
A few of the pegmatite "'bunches" show a center composed largely of
quartz, surrounded by a zone in which feldspar is dominant. In places
the pegmatite masses send off irregular and vaguely bounded
ramifications into the granite gneiss. The two types are associated in
the most irregular manner. In places the pegmatite is very coarse and
carries beryl and black tourmaline. One feldspar crystal in this
portion measured S inches across.
The
relation and mineral characters detailed above suggest the following
inferences in regard to the genesis of the rocks described:
1.
The presence of the same mineral species in the, same order of
abundance in both rocks and the many instances of complete gradation
of one rock into the other show that they are products of the same
parent magma.
2.
The fact that the pegmatite, masses in some parts of their length have
rather sharp walls and in other parts grade gradually into the granite
gneiss indicates that certain portions of the pegmatite crystallized
after some of the granite was rigid enough to develop cracks into which
the pegmatite magma penetrated, and that at the same time other parts
were fluid enough to permit pegmatite and granite to solidify with
gradual gradation and perfect crystallographic continuity between them.
3.
The intimate and small-scale manner in which the pegmatite and the
granite gneiss are associated, and the fact that these variations are
so irregular and are not related in any way to any wall rock now
observed or probably existent in the past, suggest that the causes
operative in producing the variations in texture and composition were
not of external origin, but were inherent in the magma itself.
Danville Junction.—In
the extreme western part of the town of Auburn, about 3 miles west of
Danville Junction, along the road to Poland Springs, conspicuous white
ledges of pegmatite exemplify clearly certain common relationships of
the pegmatites of this part of the State. In places this pegmatite,
grades gradually with perfect crystallographic continuity into a rather
fine-grained granite gneiss. One pegmatitic band 1 inch wide in this
granite gneiss shows contortions, which, in the absence of any
regional metamorphism later than the granite-pegmatite intrusions,
appear only explainable as the
