42 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
microcline,
and a member of the isomorphous series of plagioclase feldspars. It
should be pointed out, moreover, that if water or other gases were
present, as it is almost certain they were, they formed additional
components whose amount the analyses do not reveal, but whose influence
on the proportions of the other constituents may have been great. If
graphic granites crystallized from magmas of eutectic proportions these
were therefore eutectics of at least four components. The series of
analyses (p. 41), though suggesting that the proportions between the constituents of graphic granites are controlled by some laws,
can hardly be regarded as proving their eutectic origin. The
theoretical value of such analyses in elucidating the laws governing
rock solutions is impaired by the fact that they take no account of the
gaseous components of the magmas.
Vogt a states
that many graphic intergrowths, especially when developed on a
microscopic scale, represent the last portions of the magma to
crystallize. This fact he cites as in harmony with the conception that
they represent eutectic residues. Although this ma}' be the true
relation in some cases, in others the graphic granite was
unquestionably not the last crystallization from" the magma. In the
Fisher feldspar quarry in Topsham, for example, where large masses of
graphic granite pass gradually and irregularly into large areas of pure
quartz and feldspar, the tests of Wright and Larsen (see p. 30) have
shown that the quartz of the graphic intergrowths crystallized above
570° C, whereas the quartz of the large pure areas crystallized below
575°. The latter was therefore the later crystallization. Almost all
the gem and cavity bearing portions of the Maine pegmatites grade into
normal pegmatite containing abundant graphic granite. From the
presence of cavities and of the rare minerals, from the general tield
relations, and from the fact that the quartz of the pockets and of the
gem-bearing portions, wherever tested, is of the low-temperature
variety, there can be no reasonable doubt that these gem and cavity
bearing portions rather than the bordering graphic portions were the
last parts of the pegmatite to crystallize.''
In
considering the significance of the graphic intergrowths found in
pegmatite, it is necessary to consider not only the intergrowths of
feldspar and quartz, but also the almost equally regular intergrowths
of muscovite and quartz, garnet and quartz, black tourmaline and
quartz, etc. As muscovite, tourmaline, and garnet are-less abundant
than feldspar in the pegmatites, their intergrowths
