1 or 2 centimeters in length," represent the altered nuclei of such minerals in the hornblende rock.
The
evidence for the igneous origin of the hornblende beds on Manhattan
Island has been shrewdly and cumulatively prepared, and taken in
conjunction with the unquestioned volcanic nature of beds at New
Rochelle, Rye, Cortlandt on the Hudson, etc., and the widespread
occurrence of such intrusions elsewhere in terranes of a similar or
identical lithological facies, there is certainly scientific propriety
in referring the hornblende schists at least to an igneous source.
EPIDOTIC BEDS
Epidote
in considerable developments is found in conjunction with the
hornblende rocks of the island—seams, greenish-yellow and some inches
in width. It can be seen on the north side of West 135th Street, west
of Amsterdam Avenue. " Here," says Julien, " in the huge crumpled mass
of black hornblende schist the bright greenish-yellow seams, 2 to 5
centimeters in thickness, consist largely of epidote intimately mixed
with quartz and partly or wholly replacing both feldspar and the
original hornblende." (Fig. 23.)
THE LIMESTONES
In
the third topographical section of Manhattan Island, that extending
from 110th Street northward to Spuyten Duyvil, where the narrow and
elongated ridges swell upwards to Washington Heights and Fort George
and the Limestone Hills at Kingsbridge, we have three features of
geological interest, the limestone beds themselves, the transverse
ravines or passes at 125th Street to 130th Street and at Inwood, and
the wide, flat, alluvial drift plain known as Harlem Flats.
The
limestone beds attain an elevation of about fifty feet along the Ship
Canal, and in the cut as well as at the opening of 200th Street (Fig.
24) and in a few deserted quarries are fully displayed. It is a
glistening crystalline limestone
