22 ROCK FLOOR OF GREATER NEW YORK. [bull. 270. 
upon the island, disclose the quite striking rectilinear outlines of these 
islets. On Kemp's map in particular (fig. 2, p. 15) it is seen that there 
are two large islets and a number of smaller ones. The largest occu- 
pies the central part of Manhattan, and is roughly bounded on the 
northwest and southeast by Hudson and East rivers, respectively, 
while to the northeast and south west it is limited lrv parallel and nearly 
straight lines, which conform in direction with the southerly course of 
the Harlem. The northeastern boundary, starting from the Hudson 
shore at Manhattanville, crosses the island to the northern limit of the 
Blackwells Island gorges, just south of Hell Gate. The southwestern 
boundary begins at 32d street and Hudson River and, crossing the 
island in the same general direction as the other, would pass through 
Corlear's Hook — a salient of the island upon the south. The other 
large islet upon Manhattan comprises that portion of the island which 
is north of the Manhattanville valley, west of Eighth avenue, and south 
of Shermans Creek. These two islets miiy be called, respectively, 
the Morningside Heights and the Washington Heights masses from the 
highest [joints of each. These highest points are in each case at the 
northern ends, from which the surface of the terrane slopes gently 
to the south, probably largely because of a persistent southerly 
pitch of the rocks, as was early pointed out by Stevens. One of the 
group of small islets upon the island borders the Hudson between 
Tubby Hook and Spuyten Duvvil Creek. Another formerly covered 
some 90 acres of the middle portion of the Harlem flats, but has 
since been reduced to the narrow limits of Mount Morris. A third, 
about 30 acres in extent, once formed the continuation of Fordham 
Heights upon Manhattan, and a fourth, of limestone, measured but a 
few acres and projected from the Harlem flats at 132d street and Sixth 
(Lenox) avenue. Two other small islets were located between Fort 
George and Kingsbridge — the one on the south being composed of 
schist and the other of limestone. Blackwells, Wards, and Randall 
islands, the near-lying "sunken meadow," Flood and Mill rocks, and 
the several remaining reefs of the eastern channel should all be included 
in the list. 
COVERED PORTION OF THE ROCK BASEMENT. 
So far as the writer is aware, no serious attempt has been made to 
learn the configuration of the rock floor of Manhattan beyond what 
has been accomplished in recording a small number of scattered 
observations from borings. Gale, in Mather's report, recorded some 
23 borings to rocks, to which number Russell added 86, and Kemp, 
Gratacap, and others each added a few. 
The recent engineering developments within the area of Manhattan 
suggested to the writer in 1901 the advisability of gathering data of 
this nature with a view to restoring in a contour ma]) the submerged 
