marked, by having the longer axes of the grains of quartz and feldspar, parallel, as also those of the mica and other minerals.
In
passing (and it will be further dwelt upon) it may be remarked that
areas of the gneiss at various points over the island (Manhattan
schist, etc.) have been hardened and apparently converted into more
granitic-like masses by an injection of mineralizing material whereby
quartz lenses, and stringers of granite (pegmatite) have been
intercalated, and even perhaps a previous gneissoid mass changed into a
granite. This process, called pegmatization, may have been in part a
phase of refusion of the gneiss itself or the result of the injection
of mineral menstrua through the foliation of the mica rock.
Dr.
K. A. Lossen has, indeed, said (Einige Fragen zur Losung des Problems
der krystallinischen Schiefer, etc.) : " It is not inconceivable that
the pegmatitic aggregates represent, so to speak, the quintessence of
the gneiss, exuded into primary cracks."
The
schists (mica-schist or the gneiss) of Manhattan Island vary extremely
in texture within a few feet. It is quite characteristic for the schist
to become densely stony or feld-spathic-siliceous in texture, fine
grained and compact, brown to gray in color, and this in contact with
strips of very micaceous schist, which at a short distance again show
weathered surfaces roughened with projecting garnet crystals. Finely
individualized garnet masses are also enclosed in the schists. Such
alterations of structure are frequent in the exposed ridges of the
Ramble in Central Park. The gneiss loses its prevalent appearance in
places and becomes a granite-gneiss, wherein the stratified or
laminated texture changes to an even mixture of constituents, and
resembles a fine-grained granite. It is instructive to note the
weathering and removal of the gneiss along the faces of its bedding or
stratification.. Softer films are destroyed and the harder stand in
relief, giving a moulding-like effect. The gneiss displays jointage,
broad
