44 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
of southwestern Maine show very indefinite boundaries
and are bordered by large areas of slates and schists which have been
intruded by various amounts of granite gneiss and pegmatite and by some
granite and diorite. The contrast between the two types of contacts is
well shown within the Penobscot Bay ° and Rockland b quadrangles.
In many parts of the former area, notably along the granite-schist
contact from Bluehill village northward and from Bluehill Falls
south-westward to Sedgwick, the granite preserves its normal medium
grain up to the exact contact. In most places this contact is so sharp
that it is possible to stand with one foot resting upon typical
Ellsworth schist and the other foot resting upon normal granite. Dikes
and irregular intrusions of granite are not very abundant in the
schists near the main granite masses, and flow gneiss, pegmatite, and
basic differentiations from the granite magma are almost entirely
absent. In the Rockland quadrangle, on the other hand, the contact
relations are wholly different, the change from pure granite to pure
sediments taking place gradually through a transition zone of
contact-metamorphosed and injected sediments 2 to 3 miles in width.
These transition zones include a great variety of rocks, slate,
schist, injection gneiss, flow gneiss, diorite, diabase, pegmatite, and
granites of various textures all associated in a manner so that it is
impracticable to delineate them separately in ordinary geologic
mapping. In western and southwestern Maine these transition zones are
much broader than in the Rockland quadrangle and contain larger amounts
of pegmatite and granite gneiss and smaller amounts of basic igneous
rocks.
The
contrast between the sharpness of certain granite contacts observed ill
the Bluehill region and the very gradual transitions observed in the
Rockland quadrangle and farther southwest seem to be best explained on
the hypothesis that the broad injected zones represent portions of the
"roof of granite batholiths, whereas the sharp contacts represent the
sides of similar batholiths. The character of the rocks found in the
two types of contacts lends support to this view. The fact that water
gas and other gases and their dissolved substances escape upward more
readily than they do laterally may explain the great abundance of
pegmatite in the broad transition zones, inasmuch as the presence of
such gases is believed to be the most important factor in the
development of pegmatitic texture. It is a reasonable supposition that
basic differentiation from the granitic magma would also be more rapid
upward than laterally, and the abundance of diabase and diorite in
certain of the transition zones may thus be accounted for. The
hypothesis is also in accord with the low temperatures at which certain
portions of the pegmatites appear to have crystallized in comparison
with the temperatures of
