A seismic survey consisting of 12 reversed refraction profiles was carried out in the waters around Long Island. A 200-foot interval contour map of the crystalline rock surface is presented. This surface is approximated by a plane dipping gently south to southeast. Relief of the order of 200–300 feet is indicated. The mean velocity of sound in the basement rocks is 18,400 feet/sec. (5.6 km/sec.). Two other seismic horizons were identified, one termed unconsolidated sediments having a mean velocity of 5400 feet per second (1.65 km/sec.) and a second termed semi-consolidated sediments with a mean velocity of 6500 feet per second (2.0 km/sec.). The latter is found only to the south and southeast of Long Island.