than kaolin. Some planes are rich in glistening scales of biotite."
There
is noted a tremolitic schist at West 59th Street, between 5th and 6th
Avenues, which " is a finely fibrous, grayish white schist, resembling
closely in texture and structure the black hornblende schist in the
vicinity, but here made up entirely, to the eye, of parallel blades and
fibres of tremolite, tightly compacted, with many surfaces and division
planes stained by reddish and yellowish iron oxide."
Tremolite
and actinolite (both of these minerals, along with hornblende,
represent divergent chemical aggregates of the generalized species
Amphibole) occur at the Serpentine (ophio-calcite, p. 47) locality in
West 58th and 59th Streets, on 10th and nth Avenues. The rock here
showed various phases of aggregation, finely granular and hard to
coarsely schistose, for the most part made up of tremolite blades, with
scales of talc chlorite and needles of actinolite.
To this section belong the anthophyllites (see ante, p.
92), and in this area Dr. Julien, as early as 1878, when there was far
less occupancy of the ground by houses, made observations which
indicated the existence of two beds of hornblende rock, probably not at
the same horizon, but one above the other, bent and folded, but
interleaved with the prevalent micaceous beds. These bornblende beds
thinned out on 57th Street, thickened on 59th Street, and continued
westward, and according to this authority, the tremolite and
actinolite beds on 10th and nth Avenues were " but a facies of the
thick bed of hornblende schist between 9th and 10th Avenues."
Connected
with the occurrence of hornblende rock on Manhattan Island is the
interesting feature of its very common plication. Dana has, indeed,
observed that "the presence of hornblende or hornblendic schist appears
to have often determined a crowd of subordinate flexures and
contortions in the beds and a loss of distinctness in the minor layers.
I have explained this on the ground that hornblende is relatively a
fusible mineral, and in consequence beds that become horn-
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