GEOGRAPHIC RELATIONS.
43
with
quartz are also less abundant and are usually of smaller size. Such
intergrowths occur, however, scattered irregularly through practically
all of the coarser pegmatite masses. If the eutectic be considered, as
usual, as the residue of uniform composition and minimum freezing point
which is the last portion to crystallize, it is manifestly impossible
to regard each of these intergrowths as representing a eutectic
mixture, unless indeed several portions of the pegmatite magma are
regarded as crystallizing more or less independently of the remainder
of the mass.
MINERALOGICAL PROVENCES.
It
has already been pointed out that most, of the known pegmatites which
are rich in sodium and lithium minerals—that is, most of the
gem-bearing pegmatites—are restricted to a zone about 25 miles long and
8 to 9 miles in width extending in a northwesterly direction from
Auburn in Androscoggin County to Greenwood in Oxford County. A second
and much smaller area includes the Xewry and Black Mountain localities
in tin1 northern part of Oxford County and differs from the
larger' area in that the gem minerals are embedded in the solid
pegmatite and are not in pockets. Within both areas the
lithium-bearing phases form only a small proportion of the pegmatite
present, most of which has the normal composition. The occurrence
locally of certain masses of unusual composition is to be attributed
either to the existence in the magma of sodium and lithium in very
minute excess over their percentages in bordering pegmatite magmas, or
else to differing degrees of segregation in magmas whose average
composition was similar. As already explained, quartz associated with
lepidolite and clevelandite from the gem-bearing portion of one of
these pegmatites showed low-temperature characters, and the unusual
abundance of pockets indicates that these portions were richer than the
normal in gaseous constituents, probably mainly water vapor. In
general, therefore, the gem-bearing pegmatites were characterized by a
higher percentage of sodium, lithium, and phosphorus than the normal
pegmatites, and probably by more water vapor and a slightly lower
temperature of crystallization.
The
region characterized by pegmatites rich in fluorine minerals but not in
lithium minerals forms an area only a few miles across in the town of
Stoneham and bordering parts of other towns in Oxford County, Maine,
and Chatham. X. II.
GEOGRAPHIC RELATIONS.
The
broad geographic relationships of the granites and pegmatites are also
significant of their relationship and origin. As may be seen from Plate
I, many of the granite areas of the eastern portion of Maine are
characterized by sharp boundaries, and most of the granite areas
