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102 GEOLOGY OF NEW YORK CITY
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of
what were originally lava-like flows, after cooling and consolidation,
through subsequent molding and plasticity in the process of
metamorphism, into hornblende schists and gneisses.
His
support of this assumption rests essentially upon three
arguments—chemical composition of the hornblende rock, physical
features, and " contact alteration."
As
to the chemical composition of the hornblende rock, it was carefully
determined by analysis, and ingeniously redetermined by Julien by a
process of ocular inspection under the microscope and with the
auxiliary aid of photography (see original paper), and found to very
closely resemble the analysis of a black hornblende from Vesuvius,
Italy, of unquestioned volcanic origin.
As
to their physical features, the beds of hornblende should be dike-like,
to sustain their claim to an igneous origin, having been inserted by
protrusion along the beds of adjoining sediments. As Julien remarks, "
few continuous outcrops of our schist have reached ioo yards in length
along the strike." They appear like interposed lens-shaped masses,
thickening at the center, thinning at the edges. There are, however,
exceptions. At West 135th Street, near 10th Avenue, there was a sheet
that formerly reached from 133d to 138th Streets, or more than 1000
feet. The hornblende at Spuyten Duyvil extends to-day for more than
1200 feet. The amphibole rocks from West 54th to 63d Street at nth
Avenue covered more than 2000 feet, and these rocks, as shown above,
Julien regards as a phase of the hornblende series. Repeated outcrops
of hornblende rocks along 97th to 99th Street from river to river
suggested "the inclosure of a nearly continuous sheet of hornblende
gneiss from river to river, brought up to view on one side of every
anticlinal fold and descending on the other." Julien further says: "
Combining this evidence with that from the cross section at 58th to
59th Street, there is reason to assume the existence of at least one
interrupted sheet, here and there split up into thinner layers, closely
adja-
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