4.6 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
lization
somewhat below this temperature. It is probable that many of the
finer-grained pegmatites, crystallized wholly above 575° C.
The
theory that the graphic intergrowths in pegmatites represent eutectic
mixtures can not be regarded as proved by the published analyses.
Certain field evidence is unfavorable to the eutectic theory.
The
broader field relations suggest that the large areas characterized by
particular abundance of pegmatite intrusions constitute in reality the
roofs overlying granite bathohths. Where more extensive erosion has
exposed the flanks of such bathohths, pegmatite masses in the bordering
schists are not abundant.
LOCAL DESCRIPTIONS.
ANDROSCOGGIN COUNTY. AUBURN.
CHARACTER AND DISTRIBUTION' OF THE PEGMATITE.
Large
areas in the town of Auburn, ('specially in the valleys of Androscoggin
and Little Androscoggin rivers, are covered with sands of glacial
origin which obscure the bed rock. Wherever the latter
is exposed, however, it is found to be either quartz-mica schist or
pegmatite intrusive in the schist or a coarse gneiss resulting from a
very intimate injection of the schist by pegmatite.
Auburn Falls. —The
prevailing rock types and the relationships between them are well shown
in the river bed at the falls just above the bridge between Auburn and
Lewiston. (See p. 11 and Pl. III, B.) The purplish-gray
quartz-mica schists, which dip about 30° NE., in many places show
distinct bedding and are of undoubted sedimentary origin. They are
similar in every way to those at the Auburn reservoir. The pegmatite
masses are intruded in general parallel to the trend of the schists.
Just below the bridge both schists and pegmatite are cut by a dike of
fine-grained diabase 3 to 4 feet wide.
The
largest pegmatite mass exposed crosses the river bed at the falls,
which are a result of the superior resistance to erosion offered by
this pegmatite and its bordering intensely injected schists as
compared with the ordinary phases of the schists. This pegmatite sill
has a maximum thickness of about 20 feet and extends nearly across the
river bed, though it forks at several places. It preserves about the
same coarseness in the wide and narrow parts and in the center and next
the walls. Its contact with the schist is everywhere sharp, and there
is not the least evidence here or anywhere in this vicinity of any
absorption of schist by the pegmatite.
Auburn reservoir.—Fresh exposures of the schists were also beautifully shown at the new reservoir site on Goff Hill in Auburn. This
