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MANHATTAN ISLAND
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81
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with
reference to localities also, is as follows : " Portions of the schists
or gneisses themselves are sometimes found to have become separated and
inclosed within the margins of intrusive pegmatitic dikes, perhaps
displaying initial stages of absorption; for example, on the knoll in
Riverside Park, opposite 83d Street.
"
Much more important have been the results of interchange, in reverse
order, from igneous intrusions cut loose during diastrophic movements
from connexion with their underlying magmatic sources, swallowed up
and permanently imprisoned within the invaded schists and these now
found in various stages of shearing, alteration, and absorption.
"
Examples of the acid occlusions have been described. The amount of
pegmatitic and quartzose matter thus introduced has been so great that
huge masses of saturated gneisses have been converted, in part or
wholly, into bedded granite, well shown still at Mt. Morris Park, at
north end of Central Park, and on Morningside Heights." This language
recalls aspects of Dr. Hunt's crenitic hypothesis, though its
author would not endorse Dr. Hunt's very speculative views. Dr. Julien
contends for a " vast dissemination of occluded igneous matter" in the
schists of Manhattan Island, and apparently would refer the strongly
biotitic (biotite is the black ferro-magnesian mica) schists of
Manhattan to the assimilation of diorite intrusive dikes (which are now
supposed to be represented by the hornblende schists, see p. 96), from
which both the iron and magnesium have been derived, which enter in the
composition of the biotite mica. This theory involves the conception of
the hornblende of the diorite being altered at the boundaries of the
dike into biotite, and the dike becoming measurably converted into a
biotitic gneiss or biotite schist, and the biotite enrichment of the
mica schists, further away from the dike, being due to the hornblendic
contents of the dike. This is certainly not always the case, as can be
seen by examination of the hornblende schists (metamorphosed di-orites)
and their adjoining mica or gneiss rocks.
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