The
very micaceous rock, which might not be considered as quite typical,
from West 125th Street, near Riverside Drive, has been called by Dr.
Julien a " schistose biotite-gneiss," and was described by Professor J.
P. Iddings as having a structure " caused by flakes of mica grouped in
lines curving about larger crystals of feldspar and quartz. It was made
up of biotite, quartz, and feldspar, all allotriomorphic (which means
crystallization in coarse-grained rocks where the component mineral
crystals have irregular forms imposed upon them by adjacent minerals),
with a small amount of muscovite, apatite, and zircon. The quartz
grains are irregular, but the smaller are often rounded. The feldspar
is chiefly oligoclase, with a little orthoclase."
Dr.
A. A. Julien, examining a specimen of almost identical rock at 118th
Street, found under the microscope that it was composed of " closely
fitting grains of colorless quartz and feldspar." These two minerals
formed about 65 per cent, of the whole volume of the rock, and of these
the quartz made up 40 per cent., being angular and clear, inclosing a
few bits, scales, of hematite, biotite, brown zircon, needles of
fibrolite and minute fluid cavities. The feldspar displays
polysynthetic twinning. Biotite mica (magnesian mica) occurs to the
amount of 18 per cent., in reddish-brown plates. Muscovite mica (potash
mica) makes up about 7 per cent., and is often or usually inclosed in
the biotite. He found also sillimanite, and fibrolite (almost identical
minerals), garnet, zircon, and hematite. These latter are in minute
quantities imbedded and mingled in the interstices and in the substance
of the quartz and mica.
Class Direction.—The teacher in crushing a piece of the mica-schist
or gneiss will frequently be able under a glass to separate out some of
these accessory minerals, or at least, under a glass, show their
presence.
In such gneisses or schists, as illustrating their characterisÂtic structures, the surfaces of cleavage (schistosity) are well
