feet
of dolomite, bounded on both sides by fissures of completely decomposed
(and soapstone-like) mica schist. . . . The dip of these fissure faces
is about 22 degrees off the vertical. The strike is slightly west of
north."*
Blackwell's Island Bridge, gueiss
Williamsburg Bridge, gneiss.
East River Bridge, Thompson Street, gneiss.
Brooklyn Bridge, gneiss.
Rapid Transit Tunnel, East River, gneiss.
Well on Governor's Island, " micaceous type of gneiss " for 1,800 feet.
Jersey Flats, gneiss
" nearly vertically stratified." " On the south side of Ellis Island
piles were driven into hard bottom at depths ranging from 16 to 23 feet
at mean low water. No rock was encountered. Buttermilk Channel has been
dredged to a clean depth of 26 feet below mean low water without
uncovering rock in place."
The
shaft sunk by the Pennsylvania Railroad Company on the Manhattan side
of their North (Hudson) River tunnels was in good rock, said to be by
Mr. Forgief " of good, sound granite." Outside of this immediate edge
of the island the engineers fully anticipated a problem of some
complexity, in the composition of the soft bed-filling of the Hudson
River, as " involving different kinds of soil (quicksand and gravel,
loose earth and rock) and the transition from one to another," which
resulted in a carefully and entirely successful shield-design by Mr.
Forgie. Chief-Engineer Jacobs has testified that the sediment, or mud,
in the Hudson River " was found to be on the average about 132 feet
deep"—that is, he explained, it is about that distance from the bed of
the river to the rock formations.
Hugh
T. Wreaks, engineer of the Underwriters' Laboratories of this city,
has thus described the shore conditions along the North River:
*
Specimen of " dense basalt" taken from Man-o'-War Reef, East River, was
shown by Julien to be a drift fragment of Palisade trap.
t The construction of the Pennsylvania Railroad tunnels under the Hudson River at New York City, by James Forgie; Engineering News, Vol. 56, p. 603 et seq.
