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90 GEOLOGY OF NEW YORK CITY
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falls
beneath a strictly determinative concluson. But it is intensely
interesting and suggestive, and separated statements reopen the
question of the geological position of the Manhattan succession in a
manner, it would seem, not hostile to the views here advocated.
Professor Berkey finds a basal gneiss representing in general the whole Highlands region, and upon this towards the south his group of Inwood Limestone and Manhattan Schists gives
evidence of complete uniformity, so that "it is the writer's opinion
that there are two distinct groups of formations above the basal
gneisses in the Highlands region. The older and more complex, wholly
crystalline, at the base a limestone (In-wood), followed by a schist
(Manhattan), both of pre-Cam-bric Age, occupies together with the
gneisses almost the whole of the Highlands and the southward extension
to New York City. The younger, a Cambro-Siluric series, with a
thick quartzite always at the base (Poughquag), followed by a
limestone (Wappinger), and completed by a slate (Hudson River), forms
a continuous border along the north of the Highlands."
(FURTHER VIEWS ON THE GRANITES OF MANHATTAN ISLAND AND VICINITY AND THE SERPENTINE ROCKS
Dr.
A. A. Julien has quite extendedly studied the granitic occurrences on
Manhattan Island, and his views are embodied in a paper read before
the American Association for the Advancement of Science in 1900. The
granites on New York Island considered by Dr. Julien were the pegmatites or
coarse granites, where the separate constituents (quartz, feldspar,
mica) have a strongely marked isolation; that is, the quartz and
feldspar and mica, instead of being intimately mixed in a ground mass
of fine texture, form sensibly large areas adjoining and mingled with
each other, and, while still a granite, have a very coarse structure.
Many of the granites on Manhattan Island belong to this type.
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