Class Directions.—The
teacher should take into the classroom some characteristic example of
this rock, explain its composition and structure to the class, call
the attention of the pupils to its different constituents, the flakes
of mica lying in one way, like scales laid down in order on one
another, the gray granules of quartz and the white particles of
feldspar gathered somewhat by themselves in lines; then crushing the
fragments under a hammer, let her separate the different minerals into
piles, revealing their mutual ratio, and objectively demonstrating
that the rock is a mixture of
these separate minerals. Let her accent the difference of the two
micas—muscovite and biotite— the former a silicate of alumina and
potash with water, and light in color, the latter a silicate of
alumina, magnesia, iron and pot- ash with water, and dark in color; let her note the relative hard-
ness of all the minerals in the gniess, the soft micas, the harder feldspar, and the very hard quartz.
It is to be observed that the gneiss is regarded here as including rock varieties which Professor Kemp has termed mica schist, viz., the
very micaceous beds in which a granular cement of quartz and feldspar
occurs, interleaved with or penetrating the mica. The harder gray and
dark gneiss is less commonly seen, perhaps, than the more micaceous
gneisses, which become mica schist upon the almost complete
disappearance of the feldspar and quartz. The gneiss thus presents
greatly contrasted conditions, and they are generally dependent on the
greater or less development of mica.
Some
excellent exposures of gneiss and its mutations are to be found in the
Transverse Road across Central Park at 79th Street; at the entrance to
the Park from 8th Avenue at 106th Street; along the bluffs at 110th
Street and the Cathedral site; in the blocks of rock left standing near
Central Bridge, where there is much variegation of color; in the ridge
culminating in Washington Heights, along the Convent grounds, and
beyond, while in cellar and water pipe excavations above 42d Street it
was often seen to a great advantage, its almost vertical sheets
cleaving off in huge plates being admirably shown. Sites also in West
93d Street, West 123d Street, and between 5th and 4th Avenues at 120th
Street, Harlem, can be profitably studied. This gneiss underlies all
the island to its extreme southern point. Cozzens (1843) mentions its
occurrence on the surface of the Battery, also at the east end of 14th
Street, while in Bleecker Street a boring for water passed through five
hundred feet of gneiss early in the century. This same formation
extends below the mud
