as
an observer can determine, but regions of the Manhattan schist conform
(as Merrill admits) very closely to the Ford-ham rock in physical
appearance. Merrill separates, as is pointed out later, these two
contrasted beds into chronologically widely removed geological ages.
(See p. 89.)
The
third division of the gneiss embraces the Yonkers gneiss, not visible
on New York Island nor seen within the limits of the greater city. It
is typically developed at Yonkers and is a hard and quartzitic rock,
with biotite (mica), hornblende, garnet, zircon, titanite, and apatite.
The rock is foliated. Merrill regards it as an intrusive rock, that is,
igneous in its origin, or more simply a softened or pasty lavalike
flow, which has entered extensively the Fordham gneiss with which it is
associated.
A
rock also associated with the Fordham gneiss is the Lowerre Quartzite
which appears at Lowerre, Bronxville, and Morris Heights, and which may
have considerable significance. It is called the Poughquag Quartzite
by Merrill. It is a hard, very siliceous rock, thin bedded in most of
its exposures, white to brown in color. Dr. Merrill considers this rock
the analogue of a Cambrian Sandstone, and regards it as the base of the
sedimentary metamorphosed series of rocks on Manhattan Island.
Overlying it is the Inwood Limestone (Stockbridge Dolomite), and
underlying it is the Fordham gneiss, which the same authority regards
as pre-Cambrian or Archaean (see page 89). These distinctions and the
geological sequence of the New York Island rocks are reviewed in
another section.
The
universal presence of gneiss (or mica-schist) over Manhattan Island,
its great depth, and the varying nature of its contents have been
clearly demonstrated in the excavations made for the Rapid Transit
Subway. The broad slabs of shining micaceous schist exposed in the pits
at 42d Street and 4th Avenue and Broadway were very instructive, and
their position, in vertical sheets, showing the steep dip of the bedded
rock equally so. (Fig. 4.)
