cubic
yards, while on the other flank the spit at Sandy Hook has advanced
about a mile and is now moving into the bay, where it deposits a half
million yards on the point every year, to say nothing of the sand held
in suspension and which has been removed by dredging. The great
quantities of drift thus advancing steadily into the entrance are
becoming a serious menace to the harbor.
The
Borough of Richmond (Staten Island) is separated from New Jersey on the
west by the channel of the Arthur Kill, and on the north by that of the
Kill van Kull, which latter has a depth of 30 feet.
Manhattan
Island is limited to the west by the Hudson, which has cut a deep gorge
in rocks presumably gneissoid in character, but which Stevens
hypothetically considers limestone^—an unlikely supposition. This
gorge has been filled up by a stiff, tenacious, clay-like silt holding
recent fossils, as the common blue crab {Callinectes hastatus Fab) and shore shells (Mactra lateralis Say),
etc. The soundings for the Pennsylvania Railroad tunnel showed an
average thickness of the mud bottom to the rock of 132 feet.
On
the east the island is bounded by the continuous channel of the Harlem
and East Rivers, the upper portion of which, that north of Fort George,
is cut in limestone. This portion is bordered by low banks which widen
into a cirque-like area, which was probably an expansion of the Harlem
River which has now withdrawn into its present and marsh-invaded bed.
This immediate region has been elevated and depressed, and the
accumulations of various deposits in descending layers furnish records
of their character. Marsh land and river mud and forest beds with sandy
zones tell of its mutations.
The
East River has an average depth of 50 feet, with some remarkably
cavernous holes at Hell Gate (150 feet). The Harlem River and Spuyten
Duyvil Creek are only 12 to 18 feet deep. Rock bottom, however, is far
below.
The most important disclosure made in this region was the discovery in November, 1891, during the excavation of the
