34 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
schist
folia do not in general conform to the outline of the large pegmatite
mass, as they would if any considerable amount of softening had
occurred, still in a zone an inch or two wide along the immediate
contact such softening has taken place with a deflection of schist
folia toward parallelism with the pegmatite contact. The bending of the
schist folia in the manner shown indicates also that the pegmatite when
intruded behaved to a certain extent like a solid body capable of
exerting differential thrust on the inclosing walls of schist. In a
body behaving essentially like a liquid, pressure would be equ alized
in all directions and it is difficult to see how such bending of folia
along the borders of the mass could occur.
Another
instance of still more extensive softening of the schists bordering
pegmatite, with the development therein of minerals derived from the
pegmatite magma, was observed at llumford Falls (PL X, B). The
contact is very irregular and the schist folia near the contact curve
around so as to conform rather closely to the outline of the pegmatite
mass. Not only are there irregular protuberances of the pegmatite into
the schist, but there are developed in the schist next to the contact a
number of masses, mostly composed of feldspar but with some admixture
of quartz, which in the plane of the section are not connected with the
main pegmatite mass. There may, of course, have been some connection
between them and the main pegmatite body, either above or below the
plane of the present surface of exposure. A feature of especial
interest is the development in some of these masses of well-defined
crystal faces, as is clearly shown in Plate X, B, especially in
the mass to which the hammer handle points. The straight faces on these
masses are parallel to the cleavage directions in the feldspar and
there can be no doubt that they are crystal faces. These relations
plainly indicate a considerable permeation of the schist by the
pegmatite magma and a sufficient yielding on the part of the schist to
permit the development of very perfect crystal faces in the feldspar.
This may have been accomplished through absorption or by metasomatic
replacement of the schist, but other evidence of absorption is wholly
absent, for the contacts though very irregular are very sharp, and no
difference is noticeable between the pegmatite next the contact and
that some distance away. It seems more probable, therefore, that the
phenomena observed indicate a yielding of the schist through
recrystallization to the pressures of various kinds exerted by the
pegmatite.
Further
instances of the softening of the schists as a result of the intrusion
of pegmatite are exemplified by numerous occurrences of the type
illustrated diagrammatically in figure 1 (p. 11), where the schist
lamina; show a thickening opposite the nodes of the pegmatite dike or
sill and become thinner opposite the bulges.
