reach
the unchanged rock, as in the case of the Astoria Hotel, whose
foundations penetrate thirty-five feet into the gneiss, and the river
piers of the Harlem Bridge, which rest upon layers of rock forty-five
feet below their first surface.
Professor
Kemp has remarked the tendency of the gneiss "to break up into large,
irregular rhombohedra, or inclined prisms," producing a " step "
structure, and instances the north end of 10 th Avenue and the foot
(east) of 50th Street. The very micaceous gneiss which, in some
nomenclature, passes for mica-schist can be seen at a number of
exposures on the east side, beginning at the East River Park, 86th
Street and East End Avenue, where granite veins are present. From this
point southward, at 80th Street, with granite veins on the river's edge
with drift trap boulders; at 77th and 75th Streets and the river; at
73d Street in a moderately high bluff, east of Avenue A, and rather
more micaceous, with fewer granite veins. It can be easily followed to
70th Street, and rises on either side of the Avenue, forming at 59th to
58th Streets and at 51st and 50th Streets, the steep wall of the East
River channel, through which the tides surge tempestuously. Opposite
this last point it can be seen in hummocky islands west of Blackwell's
Island in the middle of the stream.
It weathers into a black, rusty surface, apparently becoming covered with a ferruginous (iron) exudation.
DIVISIONS OF THE GNEISS
Dr.
Frederick J. H. Merrill, State Geologist of New York, has separated the
gneiss areas in and north of Manhattan Island into three broad
divisions, which are made referable to stratigraphical and lithological
distinctions; that is, the divisions are due to a difference in age
and origin. These distinctions have been worked out by their author
with such skill and learning that no complete understanding of New
York's local geology can be acquired without their mention, though in
all respects they do not yet seem established.
These divisions of the gneiss areas embrace, first, the Ford-
