at the time of metamorphism to
make the black, iron-bearing mica, or hornblende, the rest of it
entering mainly into pyrite, and, sometimes, garnet. . . . Hornblende
has been formed where iron existed without enough of potash for making
mica."
The rocks of Manhattan Island teach the lesson of metamorphism everywhere,
and the teachers must endeavor strenuously to acquire a keen
realization of that remarkable geological process. They have otherwise
missed the great lesson the rocks of New York convey.
The
hornblende beds of New York Island, if interpreted as intrusive dikes,
are still examples of metamorphism. The structural character of the
original plastic eruptive rock has been changed, and the heterogeneous
crystalline mass becomes altered to a schistose felted stratum of
hornblende. The connection of schists with original masses of diorite,
gabbro, and diabase was pointed out by Lehman. Jukes, indeed, suggested
long ago that hornblendic and augitic lava and tuff may be
metamorphosed into schists.
DR. JULIEN'S ESSAY
Recently
a very extended study of these hornblendic rocks has been made by Dr.
A. A. Julien, which has emphasized the interpretation of these beds as
intrusives, though not casting aside as unwarrantable or untenable the
view of their possible sedimentary origin. Dr. Julien especially
studied those which are found north of 44th Street, and many localities
indicated on the map accompanying this paper are now covered over by
buildings.
Julien
points out, what has not been hitherto clearly indicated, that the
hornblende rock (quartz diorite or hornblende schist) is sometimes
found in intercalated zones, sheets, or ribbons in the island gneiss
rock, showing " more or less disturbance, flexure, and even zigzag
folding, with variations in thickness, and frequent passage at their
margins into vaguely defined and crumpled layers of biotitic gneiss."
These occur-
