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GEOGRAPHIC RELATIONS.
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crystallization
of normal granites; it also accords with the presence of numerous dikes
of very- hne-grained granite, some so fine as to be rhyolitic in
certain of the contact zones, and with their absence about the sharper
contacts.
SUMMARY.
Field
and laboratory studies of the Maine pegmatites indicate that all are in
a broad way contemporaneous and are genetically related to the
associated granites.
External
conditions, though locally having some slight influence, are not
primarily the cause of the pogmatitic textures. The presence of the
rarer elements seems to have had only a minor influence on the texture,
for in many typical pegmatites such elements appear to be entirely
absent. Theoretical considerations and the presence of miarolitic
cavities in certain pegmatites point to the gaseous constituents of
the pegmatite magmas, especially water vapor, as the primary cause of
their textures.
Although
certain facts, such as the pinch and swell phenomena observed in many
pegmatite dikes in contrast with the parallel-walled character of most
of the granite dikes, indicate somewhat greater mobility in the
pegmatite than in the granite magmas, other facts, such as the
sharpness of many of the contacts between pegmatite and schist, the
absence of absorption along any of the contacts, the presence of
angular schist fragments now surrounded by pegmatite, the small
proportion by volume which the cavities bear to the whole pegmatite
mass, the absence of notably greater contact-meta-morphic effects near
pegmatite, than near granite contacts, and the batholithic dimensions
of some pegmatite bodies, all suggest that the difference in average
composition between the granite pegmatites and the normal granites was
relatively slight and that the pegmatite magmas were not so greatly
different in physical characters from the granite magmas as has been
commonly supposed.
In his text-book on igneous rocks a Iddings,
in discussing the pegmatites, says ''the amount of gases concentrated
in such magmas was not many times that of the gases originally
distributed throughout the magma from which the pegmatite was
differentiated; possibly not more than ten times as much." The present
writer would be inclined, in the case at least of the granite
pegmatites of New England, to estimate the gaseous content of these
rocks at a still lower amount.
The
experiments of Wright and Larsen on quartz from pegmatites from Maine
and elsewhere indicate that some at least of the coarser pegmatites
began to crystallize at a temperature slightly above the inversion
point of quartz (about 575° C.) and completed their crystal-
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