MANHATTAN ISLAND 115
the East River, at least, was a tumultuous one, broken by rapids and falls.
And
in considering this hypothesis it should be recalled that the dropping
down and out of rock sections two to five hundred feet involves abyssal
conditions in the crust of the earth not to be lightly invoked. In
mountain ranges there do, indeed, occur displacements of extraordinary
dimensions, but this amounts to reshifting of elevations already above
sea-level, and accompanied with uplift and overshove crumpling and
crushing. Dr. Hobbs' view seems to contemplate a falling out of blocks
along joint walls, as, in a tile mosaic, one tile may sink below the
level of its neighbors. The memoirs and papers of Professor J. J.
Stevenson have fully displayed the wonderful extent of the faults in
Tennessee, Virginia, and Pennsylvania, with differential changes of
500, 2,400 and 2,800 to 12,000 feet, and these have largely been
considered by Professor Stevenson as " cracked anticlinals." Professor
Stevenson refers the time of these faults to the Mesozoic {Triassic).*
In
1895 the tunnel for the East River Gas Company from East 70th Street,
underneath Blackwell's Island to Ravens-wood, Long Island, was
completed. The tunnel is 2,516 feet long, and about ten feet and a half
in diameter. The rocks met were mica schist, " much contorted, but with
a well-marked general dip of about 8o° west" (Kemp), kaolin with garnet
and biotite, quartz, decomposed mica schist, a fissure filled with soft
mud, river sand, and near the east shore of Blackwell's Island ten to
twelve feet of dolomite. Beyond this, compressed in a synclinal fold,
were mica schist and again white " crystalline dolomite precisely like
the outcrop at KSngs-bridge " (Kemp). If it is assumed that this
dolomite lay in the flooring of the west channel at this point, then,
as Professor
An
item of interest to students of our Manhattan geology is thus recorded
by Professor Stevenson: " The crushing at several localities near
Clinch River is excessive, and at one locality the shales are folded as
closely as micaceous shales on Manhattan Island, but they show no
evidence of metamorphism."
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