32 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
greater
influence in lowering the freezing point, decreasing viscosity, and
affecting textures, than do constituents of high molecular weight. They
may thus attain an importance which appears disproportionate to the
small part by weight which they form of the whole magma. The substances
(hydrogen, water, fluorine, chlorine, and boron) commonly believed to
exert the greatest influence upon the viscosity of magmas and the
textures of the resulting rocks are all substances of much lower
molecular weights than silica and the rock-making silicates and oxides,
even if minimum values for the latter are assumed. The hiatus between
the molecular weights of these two groups of substances is so marked as
to justify the retention of the term "mineralizers" for the lighter
group, in case the principle outlined above is eventually shown to be
operative to an important degree in magmas.
VISCOSITY AND GAS CONTENT.
The
field and laboratory data on the pegmatites of Maine that bear on the
viscosity and gaseous content of the pegmatite magmas may be set forth
as follows. As the pegmatite magmas crystallized at some distance below
the surface, the gases which they contained must either have made their
escape through the wall rocks or else must have remained in cavities or
occluded within the solid pegmatite mass. The escape of such materials
through the wall rocks should presumably leave some record in
contact-metamorphic effects. Their retention within the rock should
presumably be recorded in an especial abundance of miarolitic cavities
and fluid or gaseous inclusions.
Miarolitic cavities.—The
field studies of the writer in Maine and other parts of New England
show that the granites are almost wholly devoid of miarolitic cavities
of any kind. An isolated cavity of small size is occasionally found,
but its walls are usually more or or less pegmatitic in texture. In the
great bulk of the pegmatites of Maine, particularly the finer-grained
ones, such cavities are also exceedingly rare. In the coarser
pegmatites, however, they are a characteristic feature, though usually
as far as can be judged constituting considerably less than 1 per cent
of the total volume of the pegmatite. Within the very narrow
gem-bearing zones of certain pegmatites miarolitic cavities may form a
considerably larger percentage of the total volume. Such cavities have
been attributed by various writers to shrinkage of the pegmatite mass
in crystallization. This may in fact play some part in their formation,
but that they are not entirely the result of shrinkage, but, on the
contrary, were filled or partly filled with some material which has
since disappeared, is shown by the presence of perfectly developed
crystals of quartz, tourmaline, and other minerals projecting inward
from the walls of
