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MANHATTAN ISLAND 65
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deposits
of the bay, crops up in Governor's Island, underlies Long Island and
Staten Island, and reaches westward and eastward as the earliest and
basal geological formation, though again itself underlaid by older rock
to the north. An attempt has been made by Dr. Merrill, formerly of the
State Survey, to separate the gneiss of Manhattan Island from those
similar rocks at Yon-kers and Fordham, a demarcation alluded to in
another section of this paper.
MICA-SCHIST
The
mica-schist on the island, if it were understood to include the very
micaceous beds of gneiss, would be almost more abundant than the gray,
harder gneiss. Where characteristically shown, it is a rock, made up
of mica plates, usually larger than the scales of mica in the gneiss,
the plates compacted and interruptedly imbricating (shingle-like),
forming a mica rock quite cleavable and almost entirely composed of
this one mineral.
There
is a natural difficulty felt in determining the mica schist and the
gneiss, at the point where they grade into each other, and become
indeterminately confused. The typical gneiss is a harder gray to white
compact rock, showing linings containing considerable quartz and
feldspar, with the mica reduced to specks. As the mica increases in
quantity the rock becomes softer, more schistose, or cleavable, and the
quartz and feldspar diminish, dwindling down almost to extinction, when
the term mica-schist becomes applicable.
Unless
cut through, neither gneiss nor mica-schist displays its structure,
weathering only into rusty-brown surfaces, usually splendent with mica,
when mica is predominant, or dull gray-streaked slopes, when the
feldspar and quartz reach a more normal development. Both mica-schist
and the micaceous gneiss easily retain water, and undergo a
disintegration which completely breaks down their coherence, so that
while retaining their original relations the components crumble with
the slightest pressure. This necessitates their extensive removal for
considerable depths, when built upon, in order to
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