74 GEOLOGY OF NEW YORK CITY
and
that large quantities are put into shape for the Croton Water-Works ";
" used also for facings on the line of aqueduct and for culverts." It
is well known that there were gneiss quarries on the island, where the
gneiss was taken out for building purposes. The granite here on the
west side was similarly extracted.
Throughout
the gneiss rock of the island granite veins occur, and their relations
to the inclosing gneiss is interesting. They can be readily recognized
at some distance as white bands, and they are of all widths, sometimes
thin strips, again broader zones enlarging into very conspicuous veins,
and they are arranged as parallel inclosures in the gneiss, looking
like white ribbons on a gray or black cloth, and again piercing the
gneiss films and beds at oblique or even right angles. They vary in
grain from a rather fine texture to exceeding coarse varieties, in
which occur broad crystals of mica, large cleavage plates of orthoclase
and abundant quartz. They form the matrix of many of the most beautiful
and striking mineral developments of the island. Garnet, tourmaline,
apatite, beryl, columbite, menaccanite, are found of rare or unusual
size in these veins, usually central in position and not along the line
of contact with the neighboring gneiss, while in one or two exceptional
instances the rare minerals monazite, chrysoberyl, and xenotime have
been met with, and their probable mineral contents yet remain far from
exhausted.
The
granite veins already suggested are referable in formation to two
classes, those which occur bedded with the gneiss, preserving a rather
complete parallelism with the inclosing gneiss, and which seem
synchronous in origin with it, and those which cut across the gneiss
layers in various directions, and seem subsequent in origin to the
gneiss itself. The conformable veins, viz., those which lie in
parallel bedding with the gneiss, are often flexed and bent with the
gneiss sheets around them, though in such cases the veins are usually
narrow. They stand in other cases in vertical partitions like white
walls between the separated gneiss beds, and again when
