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10 PEGMATITES AND ASSOCIATED ROCKS OP MAINE.
DEFINITION OF PEGMATITE.
The
granite-pegmatites, in which are found feldspar, quartz, mica, and gem
minerals, are composed of the same mineral constituents as the ordinary
granites of the State, and differ from these principally in their
greater coarseness and in their very uneven texture.
Among themselves the granite-pegmatites differ greatly
in coarse ness, some being little coarser than ordinary coarse-grained
granite and others showing single masses of feldspar or of quartz 20
feet in diameter. Their distinguishing feature is, therefore, not
coarse of grain but extreme irregularity of grain. In a granite
different grains of the same mineral species differ in size, but
usually only within rather narrow limits. In a pegmatite, on the other
hand, they appear to differ without limit, a crystal of feldspar an
inch across perhaps having a neighbor which is several feet across.
This textural feature is illustrated on a microscopic scale in Plate
II,which is a reproduction of a photomicrograph of fine-grained aplitic pegmatite exposed in the river bed at Lewiston.
Pegmatite usually forms dikes or sill-like masses in areas occupied principally by rocks of other kinds. (See p. 11.)
GEOGRAPHIC DISTRIBUTION.
Pegmatites
occur throughout the Appalachian Mountain region from Alabama to New
York and thence northeastward into Connecticut, Massachusetts, New
Hampshire, and Maine. In most
of these States they have been worked commercially to a greater or less
extent. In Maine the commercial deposits are confined largely to
Cumberland, Sagadahoc, Lincoln, Androscoggin, and Oxford counties
though pegmatites also occur to some extent in Franklin, Kennebec,
Waldo, Knox, Hancock, and Washington counties. Their general
distribution, as well as that of the granites, is shown on Plate I. on
which are also indicated the localities which have been worked
commercially for various pegmatite minerals.
Excellent
opportunities for studying the character and relationships of the
pegmatites are afforded by many of the quarry openings, by numerous
glaciated rock surfaces, and by almost continuous exposures along the
seashore. The shore in the Boothbay region especially is an excellent
field for study.*
GEOLOGY.
BORDERING ROCKS.
The
geologic relations of the Maine pegmatites show that most of them are
distinctly intrusive into the surrounding rocks, although the
conditions of intrusion are somewhat varied: and that in origin they
are closely connected with the granites (p. 27). The rocks
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