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Ch. 2: Manhattan Island

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MANHATTAN ISLAND
107
which conditions would plausibly argue a contemporary origin for both gneiss and limestone.
As to the second group of facts ( ?), there are some doubts permissible as to the construction given to the relations ob­served, inasmuch as at some points, at least, the limestone over­lies the underlying gneiss; and, as to the first, the intergrada-tion of gneiss and limestone, two sediments collected at widely separated periods would at some subsequent moment, when metamorphism, folding, and compression began, be, at much of their surface of contact, thrust into each other, and become insensibly but thoroughly mingled. Indeed, Stevens avers that " the thrust of large masses of limestone into the solid gneiss was seen, crushing and grinding the latter as it passed, show­ing that the rocks were hardened when the folding action took place and the thrust was made."
Late observations, perhaps, acceptably demonstrate that the limestone beds of Manhattan Island underlie the mica rocks, the schistose gneisses, called the Manhattan schist, while con­clusively overlying the Fordham gneiss.
As to the interstratification of gneiss with limestone, noted by Dana, sediments that would form gneiss might naturally have accumulated through the limestone and formed beds within it. It would seem that the large masses, however, of gneiss (certainly the Fordham beds) underlie the limestone, and are older. Inasmuch as the calcareous sediments and the aluminous sediments were changed to rock at the same time, and as that time was long subsequent to their first deposition, many beds of aluminous material much later than the first, might, indeed, have formed over or in the calcareous deposits. Metamorphism practically gave all these aluminous sediments the same character, though they might be widely separated in age. The contention of authors is that the great bulk— the beds of the Manhattan schists—overlie the limestone and are of a different age.
In this connexion M. A. Yeshilian has observed that at Walton Avenue and Tudor Place, near 167th Street and
Ch. 2: Manhattan Island Page of 281 Ch. 2: Manhattan Island
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