83d Steet) seem helpful in establishing a diversity and a distinction of movement.*
At
what time these changes, which folded the rocks and lithified the
sediments, began can with no certainty be established, though it was
certainly long subsequent to their deposition, and continued
intermittently afterwards. It was, perhaps, at first gradual,
progressive, and gentle, and only after the strata had assumed
solidification, and offered greater resistance was it accompanied by
ruptures and Assuring. The beds on Manhattan Island as seen to-day
indicate much flexibility. The period of mountain-making is usually
fixed at the close of the Lower Silurian and the end of the Paleozoic,
and we may for reasons of conformity consider these folds to have been
inaugurated in the former period.
In connection with this it is appropriate to emphasize the anticlinal axis
on the west side of the island and the synclinal on the east. The beds
on the west side have been rolled up into arches which later pressure
has flattened into vertical plates, and those on the east curved
downward into valleys or troughs. The area of distortion, strain, and
crumpling is lifted more into view on the west, and is depressed more
out of view on the east. It has been remarked that the granite veins
are more numerous on the west side of the island (Fig. 22) than on the
east, and this might be expected if their origin is connected with this
violent dislocation of the rocks. It would also be reasonably expected
that the consequences of distortion would be shown more deeply seated
on the east, below the crown of the synclinal or trough, and that
granite veins would be found at great depths.
The crucial question of the age of this complex of gneiss and granite is a trying one. Without circumlocution it is believed by the author that these beds are Archaean, and that the
*On
the pyramid rising from the sward in Riverside Park immediately south
of Mt. Tom is a wide vein of granite, granular for its greater extent,
on either side, but holding a differentiated center of coarse,
peg-matitic granite as a vein core. There are here some undulating and
twisted cross veins of granite.
