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172 GEOLOGY OF NEW YORK CITY
Upon
the serpentine hills as a center is superimposed, like a marginal
expansion, a skirt of later formations, which widen the narrow island
of primary hills and also prolong it into a southern terminal angle at
Tottenville. The nucleal geological feature, therefore, of the Borough
of Richmond is the serpentine hills. They form a broad belt covering,
perhaps, a superficial extent of fourteen square miles, their eastern
limit rather sharply defined by an abrupt terrace, from the foot of
which stretches a coastal plain to the sea and Raritan Bay, their
western flanks more gently sloping beneath a mantle of drift. They are
broken through at a lower level by several natural passes or cloves,
and a number of exposures afford the student and collector desirable
opportunities for comparing their mineral features.
In
a northeast and southwest direction they rise permanently in rounded
domes from the edge of the channel of the Kill van Kull and New York
Harbor and, with a fairly uniform range of elevation, extend to
Richmond, at the center of the island, where they sink rather suddenly
beneath the inundated expanse of the Freshkill meadows.
The
serpentine assumes, at but a few points, a characteristic yellowish
green, being usually pale in color and even whitish from weathering,
though tints are found quite deep and attractive. It is also almost
black, and in texture compact to earthy. Talc and unctuous surfaces are
found associated with the serpentine, and apparently its derivatives.
Collectors will find serviceable material at Pavilion Hill, a very
short way back from the water side at Tompkinsville.
The
only other massive rock at Staten Island is the so-called " trap," an
igneous rock forced upward from some deep-seated source of fused or
molten mineral matter. This trap dike, exposed in quarries at Elm Park,
a short distance from the shores of Kill van Kull, and at Graniteville
(Fig. 41), represents the Triassic rocks, the first stage of
Mesozoic time. There are no Paleozoic rocks on Staten Island in place,
and the great gap from the crystalline schists to the Mesozoic is left
vacant
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