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86 GEOLOGY OF NEW YORK CITY
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compression
and fusion, flexed, bent and broken. There were wrenchings,
overturnings, and refusions; in fact, after the beds had assumed
partial or complete mineral stability, the subsequent movements evoked
the more violent strains, and the rubbed, triturated, and parted
surfaces gave rise, under the irresistible contraction, to new fusions
which spread and penetrated through the crevices; these later
movements themselves originated and, in the opinion of the writer,
created the secondary granite veins which cut across the gneiss, as
well as many interlaminated streaks and strips of granite
(pegmatiza-tion). Not, indeed, that the secondary granite veins were
intrusive, in the sense that they were filled from below from
deep-seated magma, but that they represented refused gneiss which,
along the openings and shearing faces, recrystallized as granite.
(Fig. 20.) This is possible, as the chemical composition of granite and
gneiss is practically identical, and both exhibit an almost equal
latitude of variation. The gneiss on Manhattan Island runs through
quite a range of variation, here feld-spathic, and there full of
quartz, and again normal or micaceous.
Segregation
has been assumed as the explanation of these granite veins, by which is
implied a gradual solution of the mineral contents of the gneiss in
heated waters, and their re-disposition as crystals of feldspar, mica,
and quartz, forming granite in veins and openings. But there is no
evidence of such solution, no vesicular or cellular structure anywhere
in the gneiss in the neighborhood of these veins whence the granite
menstruum was obtained, and the even, straight edges of some veins seem
to preclude the idea of solution which would have acted unequally along
the vein margins. This latter stricture may, however, be cast aside, as
the boundaries of the conformable veins are by no means always
straight, and those of the cross-cut veins seldom, but this fact is as
consonant with the theory of dynamic fusion as with chemical solution.
Amongst the many interesting evidences of disturbance, an
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