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146 GEOLOGY OF NEW YORK CITY
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In addition to the above list, Spinel, the
oxide of aluminum with the oxide of magnesium, can be seen in dark
octahedrons in the Kunz collection from 111th Street and Madison
Avenue. Also Prehnite, a green hydrous silicate of aluminum and
calcium, in little spheres, has been taken by Mr. D. J. Atkins from
the Subway tunnel under the East River.
Fifteen
years or more ago the approach to the lawn and grounds of the Convent
of the Sacred Heart in Harlem led up a gradually inclined path or road,
reached through a gateway entrance near the present 126th Street, and
the visitor was there impressed by the vertical wall of rock in its
vicinity. This wall was a section of the hillside itself. The opening
of Convent and St. Nicholas Avenues and the cross streets from 126th to
130th Streets has obliterated, by blasting and removal, this
conspicuous landmark. The process of reduction uncovered a most
interesting thick seam of actinolitic schistose rock, which rose
vertically and dike-like between the gneissic beds, whose structure in
its neighborhood seems to have been preponderatingly feldspathic and
quartzose. Fig. 35 shows the stump of this actinolitic interlineation.
The rock is a densely felted mass of short actinolite fibers or strips
varying from a typical green to a gray-white, like tremolite, and
showing the kneaded and crowded centers of radiation of the
transversely fractured needles. The rock is hydrated, scarcely
feldspathic, and appears in dark patches hornblendic. The more bleached
fragments resemble strongly the boulders of anthophyllite encountered
superficially over the island. Of the former extension of this bed or
dike, Mr. F. A. Camp, who has collected its mineral contents and
associations, says:
"The
actinolite dike, before blasting operations were commenced, projected
from the surrounding rock to the extent of 6 to 8 feet for about 80
feet from the north side of 129th Street. The general direction was
toward N. N. E. The" projections were about 3 feet in width, increasing
as they went down. A large amount had been broken off from these
projections, as was apparent from the actinolite boulders, down
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