The area of crystalline rocks which stretches along the north shore of Long Island sound from Narragansett bay to the New Haven Triassic embayment has received almost no geological study since the state survey of C. T. Jackson,* in 1840, in Rhode Island, and that of J. G. Percival,† in 1842, in Connecticut. To the eastward, the region of Carboniferous and Cambrian strata around Narragansett bay has been quite fully described; to the northward, the central portion of Massachusetts has been shown to consist largely of altered Cambrian sediments, now in the form of gneisses, and to the northwest and west, the Triassic and metamorphosed Cambro-Silurian strata have received detailed attention, but the triangular area forming central Connecticut, with its base on Long Island sound, remains almost entirely for the future. The rocks are prevailingly granitoid gneisses, presumably of Archean age. They . . .