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Ch. 2: Manhattan Island

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MANHATTAN ISLAND                        83
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,
Ch. 2: Manhattan Island Page of 281 Ch. 2: Manhattan Island
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