serpentine were found along the west side of Amsterdam Avenue above 165th to 181st Streets.
HORNBLENDE ROCKS
The
hornblende rocks of Manhattan Island are not numerous, but are apt to
arrest attention by their dark color, which varies from a dark green to
black. The hornblende rock, when examined by a hand glass, is seen to
be composed of flattish blades of hornblende crystals closely appressed
together in its more open textures, while it grades into a really dense
and hard fissile slate-like rock, now not often encountered. It occurs
interbanded with the gneiss, and yet rather sharply separated, the
mica-schist or gneiss seldom showing any scattered or attenuated
evidence of the hornblende along the edges of the latter. Hornblende
(classed under amphibole) is a silicate of alumina, iron, magnesia, and
lime, and, if we regard these hornblende beds as originally sediments
subsequently metamorphosed, they represent layers of ferruginous and
calcareous clays. They have, indeed, been regarded as possibly
intrusive dikes of igneous rock which have undergone alteration.
The
hornblende gneiss, a flinty-looking rock, composed of hornblende, mica,
and feldspar, is not infrequently met, as at I22d Street and Harlem
Heights, at 94th Street, between 4th and 5th Avenues; hornblende
schist, with crystals on the surface, at 80th and 81st Streets and 9th
(Columbus) Avenue, now built over; hornblende gneiss, with quartz
veins, at 116th Street and Columbus Avenue; hornblende gneiss
penetrated by the Aqueduct shaft at 165th Street and Amsterdam Avenue,
at 190th Street and 10th (Amsterdam) Avenue, here enfolding garnets;
schist, with scapolite, at 93d Street and Lexington Avenue.
The
hornblende rocks may represent iron sediments; there is a large
percentage of iron in hornblende, and, as Dana remarks, " The iron of
those sediments went, for the most part,
