28 PEGMATITES AND ASSOCIATED ROCKS OP MAINE.
cally
connected in a most intimate manner with the granites. Evidence of
this is found in mineralogical similarity, in the invariable presence
of granite in all areas where pegmatite is found, and in many actually
observed transitions from one to the other. (See fig-2.)
If
we admit a genetic connection between the pegmatites and granites, it
is next of importance to inquire what evidence is afforded by the Maine
pegmatites as to the physical and chemical conditions which resulted in
the crystallization from related magmas of rocks of such widely varying
character.
EXTERNAI, CONDITIONS.
Differences
in external conditions at the time of crystallization appear inadequate
to explain the observed textural differences. This is shown by the
close association of the two types of rocks—an association already
cited as evidence of their genetic relationships. The field relations
show that in many instances the external conditions, such as the
nature and temperature of the wall rock, the depth at which
solidification took place, etc., were similar for both types of rocks.
In cases such as that shown in figure 2 the general external
conditions must have been practically identical. A similar conclusion
is justified in numerous other instances where granite and pegmatite
grade into each other, and especially where pegmatite forms
segregation-like masses wholly inclosed in granite. Conversely, the
broad, general similarity of the pegmatites over very large areas where
the external conditions were certainly not constant also indicates
that the causes of their peculiar textures were in the main internal
rather than external. It seems necessary to look, therefore, to
differences inherent in the magmas themselves for an explanation.
DOMINANT CONSTITUENTS.
The
characters shown by the Maine pegmatites accord with the evidence
obtained from many other districts in indicating (1) that the pegmatite
magmas were characterized as a general rule by the presence of certain
components in amounts larger than occur in normal granite magmas, and
(2) that to these differences in composition were in large measure due
the differences in texture. The exact nature of such differences is,
however, more largely a matter of inference than of direct field
observation.
In
the great mass of the normal pegmatite it is exceedingly difficult, if
not impracticable, to make a satisfactory estimate of the relative
proportions of the different mineral constituents. So far as can be
judged without measurements the proportions are of the same
