No. 275.] 
189 
the corner of Seventh and Lewis-streets the rock was reached at 93 feet. 
And at the corner of Seventh-street and Avenue D, it w^as reached at 
the depth of 100 feet. In these borings it is evident that the rock dips 
down on the east side of the island to about 100 feet below the graile 
level of the streets, and that about the same depth of alluvial deposits 
have accumulated, and most of them below tidewater mark. 
The third bed of alluvium is that already referred to under the name 
of the Harlem and Manhattanville valley; the form of which approaches 
that of a scalene triangle, having the longest side on its southern bor- 
ders, and extending from the village of Manhattanville on the Hudson 
to the termination of 94th-street on the East river; its shortest side from 
Manhattanville to the northern limit of Harlem village, near I30th- 
street; and the remaining side from the last named point on the north 
to the termination of 94th'Street on the south. The width of the island 
at this place is about two and a half miles, and the length of the valley 
east and w^est the same, while its extent north and south is about iwo 
miles. The strata throughout the valley are alternations of sands, loam 
and gravel, generally in strata, but sometimes in conical hills thrown 
together in great confusion; and when this is the case, it is gravel and 
pebbles rather than sand or loam. The gravel and pebbles predominate 
towards the western portion of the valley; sand and loam in the eastern. 
Boulders are less abundant in this than in most other portions of the is- 
land. 
Comparatively few excavations or borings have been made in this 
valley, consequently we have fewer data for determining the depth of 
earth coveiing the rock. In the northern part of Harlem village at the 
corner of 4th avenue and 129th-street the rock appears, and has been 
removed by blasting; and in a number of other places in the vicinity it 
approaches the surface or within a few feet of it; indeed it is inferred 
that the whole valley is a bed of alluvium comparatively shallow, inas- 
much as the rock appears from 10 to 20 feet above the surface, both to 
the north and south of the valley, and immediately on its borders. 
The extreme eastern portions, and especially the southeastern parts, 
are a salt marsh, which, along the borders of Harlem creek, approaches 
the 3rd avenue; but as few, if any, excavations have been made in it, 
little can be said of its geology. 
Besides the above mentioned alluvial beds, others are found in diffe- 
rent portions of the island, but they are so inconsiderable as to require 
no particular description. 
