122 GEOLOGY OF NEW YORK CITY
trap
of New Jersey. This escarpment on the Palisades was made more dominant
by columnar weathering or disintegration. At the same time and through
the long centuries of the Tertiary Age the edge of the continent grew
outward by deposition, and even any original Hudson gorge was
partially clogged and loaded. (Dr. Spencer's paper reveals
precipitous edges to the canon miles away from Sandy Hook, intimating
older and denser rocks than the Mesozoic or Tertiary clays, marls,
sands, etc.) Then the Hudson, upon re-elevation of the land, scoured
and cut its way out through and beyond these softer accumulations,
reasserting the former profundity of its defile.
Canons
of a similar character (Spencer) have been described by Professor
Edward Hull, of London, on the east of the Atlantic at the coastal
apertures of European or English rivers. The canon of the Congo River,
discovered by Stassano, and the canon off Cape Verde, described by Mr.
Henry Benest, are similar. These submerged river gorges, including
those revealed by Dr. Spencer himself in the West Indies and Cuba, all
present a coincidence of features so complete as to emphasize a
terrestrial identity in origin and history. During glacial times the
Hudson River valley, or gorge, disappeared, as a noticeable geographic
detail beneath a continental ice-cap.
The
East River was a far less imposing phenomenon, probably less ancient.
An interesting feature of its bed, as shown by the soundings of
engineers, is its hummocky or hilly character, as in the single
prominence, on the line of the Subway tunnel to Brooklyn, of rock, and
its immediately adjoining fissures or valleys filled with sand and mud.
Variable hardness or density of the eroded rock may explain such
facts, or again faulting can be conveniently importuned.
DIPS
AND STRIKES The entire thickness of the gneiss and schists on Manhattan
Island may be, perhaps, one thousand feet—of the limestone six hundred
to eight hundred feet.
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