seated
rocky basement which forms the substantial support of these lofty
tenements. The origin and nature of this deposit is connected with the
glacial vicissitudes which have carried from the north and the higher
portions of the island itself this mantle of debris. It has
progessively disappeared above the grade level of the streets as the
city has advanced its populated limits. A few areas yet reveal its
nature; elevated sections of drift hills, such as that at 3d Avenue
and 66th Street—now reduced for the occupancy of the Elevated Railroad
engines—but, except for the revelation of its character, made by
excavations, it would have no witness now in the lower portions of New
York.
The
recurrent opportunity of putting up great buildings, and the necessity
of placing their first tiers upon solid rock, have led to a reasonably
complete exposure of these loose beds. The succession of beds is
variable, but a general resemblance, apart from the differing
thicknesses of similar strata, is preserved. Along the river a channel
margin, and in all places where made land is found, the first surfaces
are composed of such artificially introduced material, below this
usually marsh mud, and in descending succession sands, gravel, clay and
rock.
There
are variations of such sections, and the clay beds, sands, gravel and
mud silts may be combined in changed relations with two or more
separated beds of each. A clear conception of the actual order is given
by the following list of sections which is here in part quoted from
Professor J. F. Kemp's " Geology of Manhattan Island "; in part derived
from "Mather's Report, 4th District, N. Y."; " Cozzen's Geographical
History of Manhattan Island "; in part from results published in the Scientific American, and in part from inquiry or observation:
Broad
Street—Made ground, 4 feet; yellow clay, 6 feet; gravel and sand, 19
feet; gray clay, 10 feet; 39 feet to rock. Trinity Church—Gravel and
sand, 26 feet to rock. Washington Market—Made earth, 10 feet; river
mud, vege-
