emergent
strip, the superficially thinning end of a curvi-form arch that
attains, on the State line between New York and New Jersey, an altitude
of 547 feet, and 1,000 feet in the High Torn near Haverstraw.
The
trap rock (diabase) is familiar to teachers as the "New Jersey
Bluestone," formerly used in Belgian pavement, and probably more
closely associated in their minds with " boulder drift," of which it
forms in the area of the Greater New York so large and evident a
member. " Trap," or probably diabase, is a mixture of plagioclase
feldspar, usually labradorite and pyroxene (augite), with frequent
veins of hornblende, seldom some amorphous quartz, a sprinkling of
pyrite and magnetite, an occasional mica flake, with products of
alteration, as calcite, chlorite, and serpentine. It varies in its
texture from a dense, iron-gray, flinty ringing rock to a more open,
crystallized " pepper and salt" appearance, weathering into rusty
crusts, or crumbling away in sand. It has been improperly termed
granite, to which, in no sense, either in composition or origin, can it
be referred.
Industrially
it has been in great demand, and now used for road metal its usefulness
exceeds all previous adaptations. The Palisades are its most familiar
exponent, and the low burrowing trap ridge, scarcely emergent above
the surface of Staten Island, is the declining and subterranean
extension of that escarpment.
The
trap dike on Staten Island has an interest exceeding its mere
lithological features. It indicates the presence at great depths,
perhaps, of that group of rocks—the crystalline schists and
gneisses—which seem so representative of the underlying geological
structure of Greater New York. And in this way: Professor Nason, in the
reports of the New Jersey Geological Survey, urged that the trap dikes
of that State had been forced outward through crevices originally
extending in the deeply-laid archaean rocks, vis., in rocks
similar to, if not identical with, the crystalline schists. Without
pausing to review the evidence he presents, and admitting his
hypothesis, the trap
