50 TRANSACTIONS OF THE [Nov. 14, 
there is scarcely a vacant lot. From 59th to 110th streets, Cen- 
tral Park and the region lying adjacent to it, are but sparsely 
settled; but between 110th and 130th streets there is a triangu- 
lar level valley, having Harlem at its base, and Manhattanville at 
itsapex. North again, almost to the end of the island, the rocks 
prevail, and the country is open, although building is rapidiy 
progressing, with quickened transit and local improvements. 
At 204th street, there is another cross valley and the small sub- 
urb of Inwood. The geological formation of the island has ad- 
mirably adapted it to be the site of a commercial metropolis. 
Its miles of water front are nearly all suitable for wharfage, and 
border either on deep channels or shallow ones which can be 
deepened as they become needed; while at itssouthern extremity 
is a broad, deep bay forming one of the finest harbors on the 
coast. 
Except in the upper portions, the general surface has been 
«improved ” out of all resemblance to its original exterior; but 
enough observations and descriptions remain to restore it. From 
the Battery to 16th street or thereabouts, the surface consists 
of drift, which formerly rose in irregular hills, of varying height. 
At 16th street and the East River, the gneiss first outcrops and 
runs in an irregular line across to the Hudson. ‘The gneiss con- 
tinues, covered here and there with patches of drift, until we 
reach the cross valley called by Dr. Gale (Mather’s Report) the 
“¢ Manhattanville depression,” which extends from 94th street 
and the East River to 130th street and the Hudson, as outlined 
on the accompanying map. This valley is filled in with drift, as 
is the Inwood depression further north. Between them, the 
gneiss outcrops again as a rough ridge. Beyond Inwood, the 
northwest corner is gneiss; the northeast is limestone, called 
Kingsbridge marble. 
The formation will be described in detail under the three divi- 
sions:—The Drift, The Gneiss, The Limestone. 
The Drift will be considered first, as it is of least importance 
from a structural point of view. It will be taken up under the 
divisions Alluyium and Drift Proper. 
By Alluvium is understood the very recent and still forming 
deposits of the rivers, consisting of a soft black mud which forms 
the beds of their channels and the marshes or meadows of the water 
front. There formerly was a swamp at the lower end of Broad 
street, another at the lowerend of Maiden Lane, another at Ferry 
street, where now is the leather district which to this day is 
always spoken of as ‘‘ The Swamp ;” another began at Rivington 
and extended a mile north with a width of half a mile; at the 
foot of 30th street on the Hudson was still another, now, like all 
those mentioned, covered with made ground and buildings. 
