FOBMS OF THE INTRUSIVES.
35
Such
softening effects as those cited are confined, however, to the
immediate vicinity of the pegmatite, usually to a zone a few inches in
width, and are the exception rather than the rule, most pegmatite
contacts being exceedingly sharp and free from all evidence of
softening. Absorption (except in a few doubtful instances) appears to
be wholly absent, the contacts even in the places where softening is
shown being sharp, and the pegmatite next the contact showing no
difference in composition from that at some distance away. Where schist
fragments are inclosed in the pegmatite their sharp outlines are
preserved. Contact-metamorphic effects of the pegmatite on schists are
particularly noticeable at Black Mountain in Rumford. (See p. 96.)
Forms of the intrusives.—If
the physical conditions of the pegmatite and granite magmas were
notably different at the time of their intrusion, it would be natural
to expect some differences in the forms assumed by the granite and
pegmatite masses. Though in many cases those forms are similar, there
is in general a tendency for the smaller pegmatite intrusions in the
foliates to assume the form of a succession of lenses (fig. 1, p. 11)
and for the granite intrusions of similar size to be more nearly
parallel walled. This contrast is particularly noticeable in the
Boothbay Harbor region and near Rumford Falls and is probably
expressive of slightly greater rigidity in the granite than in the
pegmatite magma and also of greater softening of the inclosing schist
by the pegmatite than by the granite magmas. The great size of certain
pegmatite masses, such as Streaked Mountain in Hebron, is, on the other
hand, suggestive of degrees of viscosity in some pegmatite magmas not
widely different from those prevailing in normal granite magmas. The
crest of Streaked Mountain was examined for mom than half a mile of
its length and the width of outcrop examined across the trend of the
ridge for about half a mile. The whole area traversed and the remainder
of the mountain as far as it could be seen was underlain almost
exclusively by coarse pegmatite, the mountain being a "boss" of this
material. The pegmatite is of the usual granitic type and exhibits no
more than the usual amount nl variation in texture and composition from
point to point. It is difficult to conceive of a mass of this size and
general uniformity crystallizing under anything like vein conditions.
With very high gaseous content and correspondingly high mobility it
would be natural to expect more differentiation both in texture and
composition. It seems probable that the viscosity of such a pegmatite
magma was not so much below that of a granite mass intruded under
similar conditions as has been commonly supposed.
Fragments
of the wall rock are very frequently found inclosed by the border
portions of the granite masses of Maine. The phenomenon is much less
common in the case of the pegmatites but was
