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MANHATTAN ISLAND 123
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The strike (viz., the
compass direction of the axis of a ridge, chain, or Cordillera) in
Manhattan Island is very near the direction of the avenues, N. 35° E.
Dr. L. D. Gale made seventy-five observations of strike, of which fifty
gave results varying from N. 25 ° E. to N. 35 ° E., making the medium
strike N. 300 E.
The dip (vis., the
inclination to the horizon of a group of beds) is generally west,
averaging within ten degrees of ver-ticality. On the west side,
especially from the city to Harlem Valley,- it is generally vertical,
and also as far east as 8th Avenue, but on the east side from 4th
Avenue to the river the dip is irregular, varying from 450 W. to 450
E. The violence of the compression of the originally horizontal beds
has practically brought them now, as seen, in an upright position.
GEOLOGICAL RETROSPECT
The
Island of Manhattan is a gneissoid ridge, modified at its northern
limits by limestone belts, and carrying over its surface an
accumulation of detrital matter in the form of sands, clays, gravels,
and unassorted debris. Its initial stages were water-deposited
sediments, the waste of some continental area north of it, together
with the formation of limestones by living organisms. Its later
stages—the process of lithification, or metamorphism—by which these
sediments became schists" and gneisses and marbles, with an
accompaniment of physical alteration, carried these flat deposits
upward into almost vertical sheets, sundered and cracked them and
refilled the fissures with granite, which may have been the more slowly
crystallizing—or crystallizing under less pressure—fused gneiss, or, in
some cases, injected veins. At the same or some subsequent time there
may have been intruded through the island gneisses or schists, a series
of diorite dikes, igneous eruptives, which were synchronous with a
widely distributed motion, disturbance, and plutonic impregnation of
the Ap-
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