136 GEOLOGY OF NEW YORK CITY
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passing like a blade through its exact center. Mr. Niven secured some
excellent large crystals northward and west. Do-decahedral garnets seem
uncommon on the island. Drift arkose from New Jersey sometimes contains
garnets of a simple type, greenish or black.
Graphite.—Mr.
F. A. Camp reports this somewhat unusual mineral from I52d Street west
of 7th Avenue. Robinson and Bailey have both found it.
Gypsum, found in the Chamberlin collection as a net-work of white crystals on gneiss.
A group of minerals known as Zeolites, from
their swelling and ebullition under heat, and all containing
considerable percentages of water, and generally silicates of calcium
or barium and aluminum, are represented in the island rock in four
species: Stilbite, Harmotome, Chabazite, and Heulandite. These
minerals are the products of alteration, and they appear upon the faces
of the gneiss in crevices or exposures, where a gradual transference to
the surface has slowly formed them. In their most characteristic
condition they are white, but the island examples are brown to yellow,
gray and red, and are deservedly admired.
They
have been almost limited in their most handsome examples to the rocks
excavated in the 4th Avenue improvement, where the tracks of the
Central, Hudson River Railroad, Harlem and Hartford Railroads were
first sunk below grade.
In
mentioning them Mr. Chamberlin, who himself first announced harmotome,
describes their aggregate appearance under stilbite. He says: " Seven
localities on New York Island have yielded this interesting zeolite,
chief of which is Harlem Tunnel and vicinity, where the minerals
associated with heulandite, harmotome, and chabazite appeared in a
series of pockets and veins, running northeastward from 4th Avenue to
I02d Street near Lexington Avenue. The stilbite, usually of a
honey-yellow color, appeared in columnar, scopiform (broom-shaped), sheaf-like, and radiated masses,
but rarely, as at Bergen Hill, in lamellar sections of crystals. Some
of the globular groups approached a bright red in color, affording a
pleasing contrast to the yellow hue of simpler forms adjacent. Among
radiated forms one specimen is nearly fifteen inches square, containing
twenty-six rosettes. At 45th Street between 1st and 2d Avenues, a
second prolific locality afforded plates of stilbite two feet square."
Harmotome, hydrous silicate of aluminum, barium, and po-
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