such
swamps as Collect Pond developed. This pond had apparently itself
undergone oscillations in its level, as Cozzens speaks of the soft mud
from its bottom being charged with salt, as if the channel waters had
entered it. The depression of land allowed the rapid currents of the
two encircling rivers to attack the drift debris, loading the inland
portions of the island, and in conjunction with its own drainage form
these beds of sand, gravel and clay which, again upon re-elevation,
were again disturbed and submerged, to be covered by later beds, until
the island assumed its present status. What striking changes may have
been produced by still greater elevations will be described in the
resume of this article. The nature of the boulders found in this
portion of the " drift" is mentioned in the accompanying paper on the
" Evidences of Glacial Action," etc.
The
increasing number of very high buildings (" skyscrapers ") in the
lower districts of Manhattan Island has resulted in enlarged and more
detailed acquaintance with the surface conditions above the floor rock,
though the general expression and succession is identical, or but
slightly varied throughout, at all the new basements. Some of these
extend downward to considerable depths. In the Commercial Cable
building (Broad and New Streets) the cellar floor is sixteen feet below
the water-level, in the Mutual Life extension (Cedar Street) the cellar
is thirty-two feet below the water-level. Messrs. Weiskopf & Stern,
engineers of the City Investing building, furnished the following
interesting statement:
"The
soil under this building is a very fine sand mixed with very soft loam,
and is almost of a uniform character from the top to hard pan. We
encountered hard pan at a depth of about 65 feet below what we term
'curb A,' which is the curb level on Church Street, at the south side
of the lot, and which curb is about 9 feet lower than the Broadway curb
in front of this building. The mean low-water level is about —17 feet
with reference to curb A, and the level of the boiler-room floor is 21
to 21 1-2 feet below curb A. The building is founded
