the
Hudson River is thus defined; the Spuyten Duyvil and Harlem and East
Rivers have been determined by joints and displacements. He thinks no "
correspondence can be established between the directions of the belts
of limestone or dolomite and of the New York water front, except
within the stretch from Kingsbridge to Macomb's Dam Bridge. Along this
line, too, the observed facts point to the occurrence of a narrow strip
of limestone dropped down between nearly vertical faults. The sections
of the Harlem River which are furnished by the bridges across it show
clearly that it is not a simple erosion valley resulting from cutting
by the stream. The bed of the stream is marked by sudden change of
level, and the Harlem seems to have chosen its course quite
independently of the position of ridges of the harder gneiss. Under
the East River limestone has been found at but two localities—under
the channel east of Blackwell's Island and in one of the drill holes
underneath the Manhattan pier of East River Bridge No. 3. The limestone
east of Blackwell's Island is enclosed between parallel fault walls,
and appears to have been dropped down along them. The numerous
occurrences, however, of gneiss and gneiss only along, in, and under
the East River leave little doubt that the main portion of the bed is
composed of this rock" (abstract). There seems a strong temptation to
accept in a measure Professor Hobbs' view. It would appear impossible
to endorse all of its implications. It cannot be well denied that there
is a cross fracture at Spuyten Duyvil, and that it may have occured in
such a way and under just such a strain as would throw the island of
New York slightly southward and downward, lowering its southern point
and invoking, as a consequence, some lateral wrenches, as the
Manhattanville cross valley, and bringing about faults in the channels
of the Harlem and East Rivers. But the deepening and extension and
widening of these river heads must have been the result of stream
erosion, and done at a time when the land was more elevated than at
present, and it seems likely that at that higher elevation the course of
