apparently
parallel, if traced down a cliff face, they deflect a little right or
left, impinging unequally on one side or the other of the gneiss. A
striking feature in most of these conformable veins is their quite
uniform width for long distances. Many of these veins appear in the
glaciated surface of Bronx Park in the ledges around West Farms. They
can be seen at a number of points in Central Park; one of these of a
considerable width, approximately conformable with the mica rock on
either side, is most conspicuous at 106th Street and 8th Avenue in
Central Park, where its mica, feldspar, and quartz are falling away in
sand and clay.
The
veins which intersect the gneiss are displayed quite generally where
there have been any excavations made, or where the higher ridges have
been blasted away. The grain or fabric of the granite varies greatly.
The big vein in West 93d Street is a very fine-grained form, and Mr.
Gilman Stanton has observed that from at least 87th Street to 95th
Street the same constitution prevails in all the granite veins, while
farther north in this same region, near Grant's Tomb, a granite vein
was highly individualized in its components, slabs of orthoclase being
taken out more than a foot in diameter. One of the narrow veins in
110th Street shows a border of interlocked mica and feldspar, with a
vein line of quartz about an inch and a half wide occupying the exact
center of the vein. As mentioned below, the line of contact between the
gneiss and granite is often sharp, but by no means invariably so, the
granite merging and mixing in the gneiss walls on either side. The
apparent straightness and even width of the veins are also often
deceiving. A closer examination reveals expansions, undulations, and
moderate swellings or constrictions. The conformable granite veins
become mere threads in places, and in cores taken from deep well
borings, as that made under the Fifth Avenue Hotel, they recurrently
appear every few feet or even inches. The line of contact of the
granite with the gneiss is often sharp, and the edge of the granite
with the gneiss feldspar against the mica—contrasting by its frequently
black
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