110 PEGMATITES AND ASSOCIATED ROCKS OF MATNE.
rocks
are mainly schists. Within the belt, however, the pegmatitic intrusions
are more numerous and are some of them of coarser texture than in the
surrounding country. If we may use the form of the smaller and
finer-grained pegmatite masses as an index to that of the larger and
coarser ones (Which are commercially valuable), the latter are
probably, for the most part, somewhat elongate in a direction slightly
east of north, parallel to the general trend of the inclosing schists
and gneisses.
Products of the quarries.—Feldspar is
the only mineral of much commercial importance at any of these
quarries. Quartz of excellent quality is present in considerable
amounts and is often saved in the quarrying process, though at present
finding but slight market. At some of the quarries tourmalines and
aquamarines of gem quality are now and then obtained. A description of
the quarries in the order of their distribution from southwest to
northeast is given below.
Mount Ararat feldspar quarries.—A
quarry from which feldspar and quartz have been obtained is situated on
the east slope of Mount Ararat, about 1 mile north of Topsham village.
The deeper part of the excavation is about 40 feet long from east to
west, 10 feet wide, and 12 feet in maximum depth. A shallow excavation
adjacent to the northwest part of the deeper pit covers an area of
about 20 by 30 feet. The quarry has not been operated for several years.
The
lower pit exposes considerable amounts of clean, white, gray, and
nearly black semiopaque quartz but shows few masses of pure feldspar
more than 3 or 4 inches in diameter. Though feldspar was the principal
mineral sought, the quartz was saved in the quarrying process and tons
of it are now piled near the pit. The feldspar is cream colored to
nearly white and is shown by microscopic examination to be principally
microcline, with occasional very small amounts of the white soda
feldspar, albite. In the upper and shallower portion of the quarry the
amount of pure quartz is less and the amount of pure feldspar is
greater than in the lower portion. Some of the masses of pure feldspar
there are 3 to 4 feet across. They grade into a coarse graphic
intergrowth of quartz and feldspar and the latter into extremely fine
graphic granite. Only the pure feldspar and the coarse graphic granite
were used commercially. Of the iron-bearing minerals which would injure
the quality of the feldspar for pottery purposes, black tourmaline is
almost entirely absent and black mica (biotite) is rare. Garnet is
rather an abundant constituent, but is associated mainly with the
muscovite and with the finer-grained portions of the pegmatite, and
only rarely with the more feldspathie parts that are commercially
available. Magnetite occurs rarely in small irregular octahedra.
Muscovite
or white mica is also an abundant constituent of the pegmatite as
exposed in the upper pit. It is pale green to nearly
