it was a chemical precipitate resulting
from the interaction of soluble silicates and chloride and sulphate of
magnesia; fourth, the obsolete notion that it was an extruded mud
forced outward through the earth's crust.
But
an examination of a number of microscopic thin sections of the island
serpentine taken from distant points proves that it at least has, on
Manhattan Island, originated in an altered hornblende. The sections
showed the characteristic curdled, shreddy, and broken appearance of
serpentine, and reveal, between crossed Nicol's prisms, luminous
colored spots and crystalline fragments of hornblende. There seems left
little room for doubt as to the origin of the serpentine in question
as coming from hornblende masses, and we may regard the greater part,
if not all of it, as a derivative product, resulting from altered
crystalline metamorphic rocks, generally referable to the amphibole
groups.
We thus add another consideration to the establishment of a community of origin for the underlying rocks of
all sections of the Greater New York: the crystalline schists
representing a nexus of geological vicissitudes, synchronous and
identical.
The
serpentine nucleus of Staten Island has been regarded as an area of
eruptive rock, and thin sections of the rock, from an outcrop near,
Martling's Pond, a mile northeast of Castle-ton Corners (Four Corners),
and near the turnpike that traverses the island from Clifton to
Linoleumville on Long Neck, have revealed the presence of olivine, a
very common mineral element in basaltic rocks, in diabase, gabbro, and
related igneous dikes. The detection of olivine is rare, and while it
is a fair inference that the serpentine is identical in its origin
throughout, the deduction from this sporadic occurrence that the
Staten Island serpentine is derived solely from igneous rocks may be
questioned. Dr. D. H. Newland, who made this discovery, also remarks
that near Castleton Corners, " where, in excavations for the
foundations of a building, the rock has been exposed to a depth of
several feet," the presence of shining bronze prisms " imbedded in a
ground mass of dark
