the
basic dioritic or gabbro-like intrusions—if the hornblende rocks and
the altered amphibolite of the Manhattan area are so to be classed—have
remained apparently their contemporaneous associates. (See pp. 97, 99.)
The
many small meandering veinlets, mentioned above, which cross the gneiss
or schist lamination and penetrate it in looped and bending streaks,
were, perhaps, more slowly exuded from granitic magmas, and, as a
group, are referable to the later stages of the complex's history. And,
as has been suggested, they are also conceivably synchronous and
congenital with the foldings which have fused the rubbings and schist
walls, along cracks, into granitic intercalations.
FOLDS AND PLICATIONS
Perhaps
the most suggestive features in connection with the rocks and geology
of Manhattan Island are the remarkable" dislocation, crumpling folds,
and angular plications of the gneissoid rocks, a disturbance in which
the granite veins seem to have participated, or which, indeed, it would
seem, in many cases, was itself the cause of these secondary or
intrusive volumes. These folds vary from wave-like arches, with an
amplitude of inches or several feet, exhibited sometimes in delicate
ripple-like curves, or in broad folded zones, or in sharp roof angles
when the compressed beds are flattened almost into verticality. In many
cases the outer or surface crests of the arches have been weathered or
planed away, and the dome is not seen, but only the inclined convergent
layers of rock. These folds' are almost invariably steeper on one side,
vis., are pushed over, and the occurrence is noted of the fold
being thrust violently over so as to assume horizontality. These minor
examples may be considered magnified and carried to their extreme
geological consequences in the ridges of the island, the north and
south folds which represent its present relief, and are the heaping
up, through contraction, of extended beds: a contraction slowly
inaugurated and, perhaps,
