|
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
|
|||
|
|
|||