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12 PEGMATITES AND ASSOCIATED BOCKS OF MAINE.
mation
of the Penobscot Bay region." As the last great dynamic metamorphism
which affected southern and central Maine took place probably near the
close of Ordovician time, these dynamically metamorphosed sediments
are probably not younger than Ordovician.
Igneous foliates.—Others
of the foliated rocks with which the pegmatites are associated are
probably primary or How foliates; that is, igneous rocks that owe their
foliated structure to differential movement within their mass before
complete solidification. To this class probably belong many of the
foliates in the Boothbay Harbor region and about Brunswick and Topsham.
Many of them are very similar in their general appearance to foliates
of sedimentary origin but upon microscopic study are found to be
indistinguishable in mineral composition from igneous rocks. One of the
most instructive
exposures of a foliate of this type occurs on the east shore of St.
George River, near the extreme southern edge of the Rockland
quadrangle, where porphyritic granite of normal composition, with
feldspar phenocrysts from one-half to three-fourths of an inch in
length, contains a number of elongated parallel lenses of much finer
grained rock of dioritic composition. (See Pl. V, A.) The
largest of these lenses is about 6 feet long and 1 foot wide. The
inclosing granite shows a decided grain parallel to the direction of
elongation of the lenses, and in other similar occurrences in
this region the feldspar phenocrysts of the bordering granite show a
tendency toward orientation with their long axes parallel to the axes
of the basic lenses. Within a few rods of this exposure occurs another
which presents the appearance shown in Plate V, B, the
light-colored bands having about the texture and composition of normal
granite and the darker bands being quartz diorite similar to the lenses
at the exposure shown in Plate V, A.
Under
the microscope the dioritic and granitic bands air both seen to be
feldspathic and of interlocking granular texture without any
cataclastic structures. The basic bands, however, besides being finer
grained than the others, contain a much larger percentage of green
hornblende and a smaller percentage of quarts:. Both phases contain
abundant titanite in grains, many of which show well-defined crystal
form. The feldspar in both has the composition of oligoclase-andesine.
It seems evident that the gneiss of Plate V, B, represents
merely the next step of the process of combined flowage and magmatic
differentiation which developed the relations shown in Plate V, A, and
that the two represent two stages in the making of a flow gneiss. At
the time when the whole mass was in a molten condition the basic
portions were presumably more fluid than the acidic portions, and the
process is probably to be regarded as an intimate intrusion of
a Penobscot Bay folio (No. 149), Geol. Atlas C. S., V. S. Geol, Survey, 1907.
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