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Ch. 2: Manhattan Island

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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.
Ch. 2: Manhattan Island Page of 281 Ch. 2: Manhattan Island
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