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MANHATTAN ISLAND
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71
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faces
of it being divided by cracks and rifts into rhomboidal blocks. This
can be seen conveniently in the hillock of gneiss bordering the path
leading out from the cave in the Central Park Ramble.
In
this connexion the so-called Poughquag quartzite, although a gneissic
sandstone, but containing mica, and affording a somewhat schistose
structure, may be mentioned. It is characteristically developed with
the Yonkers gneiss, and can be seen in Lowerre in typical condition. It
is, however, found within the limits of the city, at Morris Docks on
the Harlem River. (Fig. 5.) It is referred by F. I. H. Merrill to the
Potsdam sandstone, and therefore represents the Cambrian formation.
(See p. 4.) Where originally described at Yonk-fers it is white, at the
Morris Dock exposure it is seen to be gray to brown gneissic in
structure, indeed, a fine-grained -sandstone, very compact, becoming a
quartzite; the schistose portions are lined in parallel position by
black specks of hornblende and by mica scales, much of the rock
feldspathic and glistening with mica. Recently the Lowerre Sandstone
and the Poughquag Quartzite have been separated. (Berkey.)
A great deal of stress has lately been laid upon the possible significance of the biotitic schists
of the Manhattan series by those inclined to give a marked expression
of vulcanism to the latter's origin,' history, and modifications. The
larger part of the mica in the Manhattan rock is muscovite, but there
are well-developed occurrences of biotite in force, which Julien refers
to as " sheets of glistening black biotite schist or biotitic gneiss,
often garnetiferous." Biotite is a mica pre-eminently associated, or
rather constitutionally involved, in volcanic rocks (it is an
orthosilicate of potassium, aluminum, iron, and magnesium) , and, as
Rosenbusch tells us, is " in the eruptive rocks one of the oldest
secretions, being formed immediately after the ores, zircons, and
apatites, which minerals are frequently included in the biotite." On
Manhattan Island much of its development is assigned by Dr. Julien to
an alteration of hornblende during shearing, the hornblende itself
representing a
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