He
also showed specimens of serpentine from boulders found in the
excavations for the Stock Exchange building on Broad Street between 40
and 60 feet below the surface.
In
the discussion of this group of formations we find that it consists of
two geological members, a series of laid beds which have been deposited
by running water, or laid down under water in some way, and a
subsequent superficial and heterogeneous aggregate of drift material
which has been accumulated by ice action. The sand and clay beds
represent the results of water action, re-assorting drift material from
some higher level; or, indeed, they may be a deposit formed from the
products of decomposition of the island rocks before the Ice Age was
initiated, while the top river mud found along the margin sinuations,
pond bottoms and estuarine levels is a distinctly modern or alluvial
deposit.
This
alluvial deposit has a further extension in the mud now forming the
bottom of the Hudson River, a deposit which has increased since
cultivation of the drainage basins in the northern part of the State
began. The opening up and tilling of the ground and the loosening of
the formerly forest-covered soil has greatly increased the amount of
earth carried away in rains and freshets and deposited in the Hudson.
The
beds of clay and sand vary in extent and thickness, as might be
expected from any sediment deposited under water, and fluctuating in
the rate of its deposition at different points, or unequally
accumulating at different times. In some sections the clay appears
absent ; never the sand or gravel. There thus seems to be deposited
over the eroded and hollowed-out edges and inequalities of the
underlying rock in lower New York, from forty to one hundred feet
beneath the surface, a blanket of clay and sand which has a very
considerable thickness. The sand beds are of importance where they
reach an extraordinary depth. The foundations of the St. Veronica
Church in Christopher Street, between Greenwich and Washington
Streets, were laid in sand, and the quality was of sharp sand, vis., such as has been deposited before being subjected
