hobbs.J EXPOSED PORTION OF THE ROOK BASEMNET. 21 
many engineers who have so courteously responded to his appli- 
cation for information in their possession. To mention all of them 
would require more space than is here available for the purpose, but 
acknowledgment should be made especially of the assistance rendered 
by the following gentlemen: Mr. George S. Rice, acting chief engi- 
neer of the rapid transit commission; Mr. J. Vipond Davies, con- 
sulting engineer; Messrs. Richard S. Buck, H. A. La Chicotte, Martin 
Gay, and Edward A. Byrne, of the department of bridges of the city 
of New York; Mr. Alfred P. Boiler, consulting engineer; Messrs. A. 
Noble and Charles M. Jacobs, chief engineers, and their assistants of 
the Pennsylvania, New York and Long Island Railroad Company; 
Mr. Wm. R. Hutton, consulting engineer; Lieutenant-Colonel Ray- 
mond and Capt. Edgar Jadwin, of the Corps of Engineers, U. S. 
Army; Mr. Cory don T. Purdy, chief engineer of the George A. Fuller 
Company; Mr. John Goodell, formerly editor of the Engineering- 
Record; Mr. Oliver W. Barnes, civil engineer; and Mr. F. L. Chase, 
engineer of bridges, New York Central and Hudson River Railroad 
Company/' The writer is especially indebted to Mr. William E. 
Brown, a member of the class of 1905 of the University of Wiscon- 
sin, who has prepared the illustrations of this report from the original 
data. 
The waterways immediately surrounding Manhattan Island are, as 
is now well known, rock cai^ons having a depth to rock of nearly 200 
feet in East River, and of 300 feet or more in North River. These 
canyons are now partly filled with drift deposits — bowlders, gravel, 
sand, and cla3 T — and silt. The depths to which the channels have locally 
been tilled by this material are to some extent dependent upon the 
velocity of the tidal currents. At Hell Gate, where these currents are 
at a maximum, the channels have been scoured out to a depth of over 
150 feet, though the average depth of water in the eastern channels 
ranges between 30 and 60 feet. In the Hudson channel, west of the 
island, the water depths vary between 30 and 150 feet, and the under- 
lying beds of detritus have been penetrated to depths of 150 to 240 
feet without meeting rock. 
FORM OF THE ROCK PEDESTAL OF MANHATTAN. 
EXPOSED PORTION OF THE ROCK BASEMENT. 
Doctor Gale's statement that the island of New York is composed of 
u gneissoid islands 11 separated by low areas fitly describes that portion 
of the rock basement which projects above the drift. The maps of 
Mather 6 and Kemp/' which outline the areas of drift and alluvium 
aOther acknowledgments are made on page 23 and in connection with the tables of Pari H. 
^Mather, Wm. M., Geol. New York, pt. 1, 1843, pp. 501-625. 
oTrans. New York Acad. Sci., vol. 7, 1887, plate facing page 64. 
