cent,
or perhaps two horizon planes in the Manhattan series, over which this
basic material has been spread or throughout which it has been
injected. In other words, a large number of these outcrops plotted on
the map after all represent but one or two intrusions."
Again
a natural objection to these hornblende rocks being regarded as
intrusive dikes is their generally exact lamination in conformity with
the enclosing rocks, and then folding apparently in unison with these
latter. Arms (apophyses), however, of the hornblende rock projecting
into the micaceous gneiss have been found. (See Morningside Park and
near 119th Street.) It is quite clear, however interpreted, that the
hornblende rock shared equally with the mica rock around it in the
curvature plication and lateral compression which attended the
upheaval of the latter from its horizontal position into vertical anti-
and synclinals.
Julien
continues his argument with apparent Evidence of " contact alteration,"
by which is meant changes in the enclosing rocks of an igneous dike,
produced by its heat and mineralizing agency. If such contact
alteration could be established it would reinforce the view taken,
that these hornblende rocks are intrusive. Julien refers to the very
evident cleanness of edge of these hornblende layers, their
delimitation from the mica rock about them being clearly and
impressively sharp. There is generally absent any convincing proof of
contact alteration, but Dr. Julien claims that " a recent
reexamination of sheets of hornblende schist, 2 to 18 inches in
thickness, at the northeast corner of West 186th Street and Wadsworth
Avenue, revealed an abundance of biotite and also of garnet, up to 1
centimeter in diameter, both within the hornblende schist and in the
contiguous pegmatitic gneiss, but only within a distance of 2 or 3
centimeters from the contact line." This seems to deserve some weight.
In
rock dikes there are apt to be coarse crystallizations, and Julien
supposes some very indefinite " obscure dark blotches or flattened
flakes, of rhombic, rectangular, or ovoid outline,
