10 -ROCK FLOOR OF GREATER NEW YORK. [bull. 270. 
sections of the surface exposures of rock. These series of sections tire 
on file in the office of the commissioner of public works. The New York 
City folio " has supplied detailed topographic, geological, and surficial 
geological maps of the area in this vicinity. 
ENGINEERING ENTERPRISES WHICH HAVE PIERCED THE ROCK 
FLOOR. 
It is safe to say that at no time in the earth's history has there been 
such a focus for engineering enterprises as is now to be found within 
the 50 or more square miles included upon Manhattan Island and its 
water fronts — enterprises which are estimated to cost several hundred 
millions of dollars. The subway and the tunnels of the rapid transit 
commission; the East River bridges, Nos. 2, 3, and 4; the proposed 
tunnels of the Pennsylvania, New York and Long Island Railroad 
Company from Weehawken, beneath the island, to Long Island City, 
with the extensions of the system in Brooklyn and the Bronx, includ- 
ing the projected bridge from Long Island City to the Bronx across 
Wards and Randall islands; and the United States Government's dredg- 
ing of the Buttermilk and Ambrose channels and of the Man-o'-War 
and Diamond reefs are some of the larger of these enterprises. While 
these vastly outweigh in importance all engineering undertakings 
that have previously been carried out within the area, the earlier Gov- 
ernment work in improving the channel at Hell Gate by removing 
Flood and Mill rocks still ranks as an engineering enterprise among 
the first of its kind in the world. The Brooklyn Bridge, the Croton 
and the new New York aqueducts, Dyckman's cut in the ship canal at 
Kingsbridge, the Easl River Gas Company's tunnel from Manhattan 
t > Long Island City beneath Blackwells Island, the Jersey City-New 
York tunnel beneath the Hudson River, and the many bridges which 
now span the Harlem River must all be included in the great engineer- 
ing undertakings which have facilitated the work of the geologist 
within the New York City area. These enterprises have together fur- 
nished more than 35 sections across the rivers forming the water front 
of the island, many of them revealing the nature of the subjacent rock, 
and not a few giving nearly complete sections across it. 
The present is, then, an especially favorable time to study the geo- 
logical structure of Greater New York, and it is perhaps nowhere else 
so important that observations be made and recorded at once lest the 
opportunity be forever lost. Not only do the great engineering under- 
takings above referred to make the present an especially favorable 
time for study, but the enormous increase in the value of real estate 
upon Manhattan Island i> resulting in a paring down of all rock 
a Geologic Atlas U. S.„ folio 83, U. S. Geol. Survey, 1902. 
