Field notes and specimens, obtained during the construction of a water tunnel in The Bronx, New York City, in 1929 to 1933, indicate that the layers of hornblende schist (amphibolite) in the Manhattan Formation are not intrusive sills or lava flows, as they have previously been regarded, but represent strata or lentils of sedimentary origin in a metamorphosed sedimentary sequence. Petrographic studies suggest their derivation from calcareous beds. Their chemical composition, like that of most amphibolites, corresponds fairly closely to the composition of basaltic rocks, but gives only inconclusive evidence of their origin. Possibly, but less probably, they may have been derived from mafic tuffs.

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