area,
and that the crystalline rocks underlying it are not met at the
surface, and that their nature and contents must be determined
elsewhere. It is in the second section, or that portion of the island
generally south of noth Street, where all the rocks of the island, with
the exception of the Kingsbridge limestone, are typically shown, and
which we are now to consider.
In
this section we find that while drift is a prevalent surface formation,
the underlying rock is also seen, and has formed numerous and high
ridges before it was leveled by municipal requirements. This rock is Gneiss, the
omnipresent rock of the island showing varieties and contrasts in
appearance, and carrying within it associated rocks, bearing a wide
range of minerals, and exhibiting the singular effects of compression
in its folds and plications.
The
term " Gneiss " embraces an extension of applications to many
mineralogically varied rocks, in all of which, however, the stratified—layer like—character
is conspicuous. The teacher taking up a large, smoothed fragment of
gneiss, or mica-schist rock, or noting its appearance in any broad
exposure, will be struck at once by the lined or banded structure. It
presents a streaked appearance, and this leaf-like arrangement of the
minerals, their juxtaposition, as it were, in sheets, is its character.
So that the word gneiss initially indicates structure, which is further
revealed in its schistosity, the property of splitting in slabs or plates.
With
this generalized application the gneissoid rocks on Manhattan Island
may be grouped conveniently thus: Gneiss (proper), Mica-schist,
Hornblende gneiss, and Hornblende schist, with a gneissoid intermixture
of limestone and mica, mentioned below.
Gneiss,
as found on Manhattan Island in most cases, is a laminated granite
usually, in its mica-schist section, having a larger percentage of mica
than granite, a smaller percentage of feldspar, and quartz in about
equal amounts, grading again into a very quartzose or feldspathic rock,
with the mica sen-
