color—of
the gneiss. Allied to these granite veins are lenses, or intercalations
of quartz, or quartz and feldspar, or feldspar alone, which interrupt
the surface of the gneiss sometimes in long ribbons, or else pinched
out into short lengths. These interleavings, knuckles, and balls of
granite are often exposed in blasting away the gneiss. They are formed
in place, embedded in the schist, and frequently they mottle the
surface with streaks of granite, which merge into quartzose gneiss, as
if it were only a phase of re-arrangement of the gneiss itself. Their
hardness has resisted weathering, and they stand out like mouldings,
and are easily traced by the eye from a considerable distance, as
those on the rock slopes of Morningside Park as seen from Manhattan
Avenue.
The
second class of granite veins is those which cut across the mica-schist
or gneiss and sometimes are seen intersecting other granite veins. They
are less uniform in width, expanding and contracting and disappearing,
in some instances, in reduced or vanishing strings, suggesting the
filling of crevices or cracks produced by shock. They are often curving
ribbons, like a drawn-out ringlet, seen on the face of the gneiss.
A
cross vein of granite could formerly be seen in West 93d Street,
between the Boulevard and Riverside Drive, transverse to the foliated
gneiss, appearing as a sinuous and quite Seven patch of white across
the gneiss for a hundred feet, with an average width of a foot and a
half. The cliff is now largely destroyed on the north side of the
street, though the vein on the south side (Figs. 6 and 7) can yet be
descried sharply angulated or bent at one side.
Another
vein was formely visible, running vertically up a face of gneiss at the
entrance of the grounds of the Convent of the Sacred Heart from 126th
Street, a very striking and impressive example. Again, a third
conspicuous vein running oblique to the gneiss, uniform in width and
manifesting something of a dike-like character, is seen on the
Speedway, some yards from its southern entrance, while others, more
conformable and wider, are seen below Fort George. The mica leaves
