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184
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GEOLOGY OF NEW YORK CITY
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serpentine
" resembles a decomposed or altered gabbro rock. The same author, in
studying the change from hornblende (amphibole) to serpentine, which
change is universal in the island serpentine, describes the process in
a very interesting manner: " In the first stages minute veinlets of
serpentine are developed along the cleavage planes of the amphibole at
the ends. From here they gradually extend toward the center, at the
same time reaching out into the lateral prisms, and widening out until
the crystal is entirely altered. By this process serpentine planes are
developed with uniform arrangement parallel to the axis of the
amphibole. In transverse sections the plates are divided into two sets
which cross each other at angles corresponding to the hornblende
cleavage."
Chromite,
magnetite, chlorite, talc, tremolite, and iron sesquioxide occur in the
serpentine. Magnesite, a carbonate of magnesia, has been found on
Staten Island in connection with the serpentine; asbestiform serpentine
is common, and the collector will note at its various exposures much
variety in texture and color, density and physical appearance, of this
remarkable lithological feature in Richmond Borough.
Pavilion
Hill, back of Tompkinsville, Grant City, the hill at Garretson's,
exposures on the road from Richmond to Spring-ville, the cut at the top
of Bard Avenue, West Brighton, the hill west of Clove Valley, and at
points near the Hessian Brook and at Fort Hill, Jersey Street, New
Brighton, are localities where the serpentine can be readily collected
and studied.
The
topography of Richmond Borough, its extreme interest and beauty, and
the combination in it of scenic features quite absent from all the
other boroughs of the Great City, will appeal to the most casual
observer. The coastal plane—or rather the covered edge of that flat
area stretching out to the rim of the continental plateau some hundred
miles beyond Sandy Hook—extends southward like a shelf from the foot of
the serpentine hills in the eastern section of the island's southern
coast line (from the fort at the Narrows to Great
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