MINERAL AND CHEMICAL COMPOSITION. 15
with
much greater range in size among individuals of the same mineral
species. Oligoclase and micrographic intergrowths of quartz and
feldspar are also more abundant in the pegmatite than in the granite.
For detailed descriptions of other instances of gradations or close
relationship between granite and pegmatite the reader is referred to
locality descriptions of the Rumford Falls region (p. 94), Stow (p.
102), Edgecomb (p. 64), Boothbay Harbor (p. 67), and the South
Waterford mica mine (p. 103).
The
granites and associated pegmatites are the youngest known rocks
occurring in notable abundance within the State. Here and there,
however, they are cut by younger small dikes of diabase, usually
aphanitic and sharp walled. Usually these occur only as individuals,
but on the shore of Keewaydin Lake (Lower Stone Pond), near the village
of East Stoneham, schist and associated pegmatite are intruded by a
remarkable network of fine-grained diabase. (See PI. XVI, A.)
AGE.
The
field studies in Maine have afforded no evidence of great diversity in
age among the, pegmatite deposits. Although all of them are not
strictly contemporaneous, it seems probable that all were formed within
the limits of a single period of geologic time. As it has been shown
(p. 27) that the pegmatites are broadly contemporaneous "with the
granites with which they are invariably associated, the age of the
pegmatites may be inferred from that of the granites.
The
evidence thus far available indicates that all of the granites of the
State are of approximately the same geologic age. In the Penobscot Bay
region granite is intrusive in rocks of Silurian (Niag-aran) age.a In the Perry Basin,6
in the extreme eastern part of the State, granite pebbles are absent
from the late Silurian sediments but are present in the conglomerate of
the Perry formation, which is probably of Upper Devonian age. The
granites were therefore intruded in late Silurian or in Devonian time,
and the pegmatites are also probably of that age.
GENERAL CHARACTER. MINERAL AND CHEMICAL COMPOSITION.
Mineral constituents.—The
pegmatite deposits in all parts of the State show great similarity in
their principal minerals, although exhibiting notable differences in
their minor constituents. Essentially they are coarse granites, their
principal light-colored constituents being potash and soda feldspars,
quartz, and muscovite,
• Penobscot Hay folio (No. 149), Cool. Atlas U. S., C. S. C.eoi. Survey, 1907.
'Smith, 0. O., and White, David, Geology of the Perry Basin: Prof. Paper U. S. Geol. Survey No. 35, 1905.
