an
additional twelve miles and to a depth of 9,000 feet at a distance of
71 miles from the head of the submarine channel near Sandy Hook. The
canon is double, the upper part being four miles wide, while the inner,
lower, more sinuous portion is less than two miles across. The period
of great elevation, probably coincides with that of the early
pleistocene. Since that time there has been a subsidence to somewhat
below the present level, followed by a re-elevation of 250 feet, as
seen by the shallow channels of the continental shelf. The region is
now sinking at the rate of two feet a century, and is underÂgoing other
and less important changes."
ROCK SOUNDINGS FROM HOBBS' ESSAY
(ORIGIN OF THE CHANNELS SURROUNDING MANHATTAN ISLAND, NEW YORK)
Spuyten Duyvil Bridge, Harlem River, rests on piles; wash borings to rock, 94 to 124 feet.
Washington Bridge, Harlem River, middle pier sunk by caissons to " an irregular rock, partly gneiss, partly marble, with veins and pockets of very hard quartz."
High Bridge, Harlem River, three central piers on limestone.
McCombs Dam (Central) Bridge, piers on gneiss.
New York Central Railroad Bridge, piles go down 120 feet, but do not reach bed rock.
One Hundred and Forty-Fifth Street Bridge, piers (4) on limestone, (1) on hard pan.
Rapid Transit Tunnel, through limestone.
Park Avenue Railroad Bridge, gneiss.
Third Avenue Bridge, gneiss.
Second Avenue Bridge, no rock encountered.
Willis Avenue Bridge, boulders and limestone.
One Hundred and Twenty-Fifth Street, gneiss.
Projected Hell Gate Railroad Bridge, gneiss from near surface to 100 feet below mean high water.
Hell Gate Reef, gneiss, as on Ward's Island.
East River Gas Company's Tunnel (Mr.
Davies' report), " Under the New York end of the tunnel is highly
micaceous gneiss rock. Just outside of the pier line it intersected a
fisÂsure. . . . Under the east channel is a seam of about 350
