The common distribution of hornblende schists of undetermined origin, exactly or essentially like those of Manhattan island, along the whole Appalachian belt, in the Adirondack region of New York, throughout the crystalline rocks of Canada and the Northwest, as well as in Europe, has led me into a rather full discussion of the genesis of our local rock, in the hope that it may satisfy a more general interest. Although nearly twenty years have elapsed since the careful investigation of similar schists in Maryland, we are still in need of “additional light upon one of the much-mooted questions of Archean geology, namely, the origin of lenticular beds of hornblende rocks so often inter-bedded in the old gneisses.”* A majority of the outcrops of this rock on Manhattan island have been covered by buildings, but many of the finest exposures (at least thirty-five) still remain open in street cuttings . . .

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