Sixty new seismic refraction measurements on the Coastal Plain of North Carolina and South Carolina were made to fill gaps in existing well and geophysical data and to make a more comprehensive study of the nature and structure of the Coastal Plain floor. Thirty-nine measurements were made on the adjoining Piedmont province to determine seismic velocities of specific rock types, thus aiding the assignment of lithologic types to basement velocities observed on the Coastal Plain. Seismic velocities observed for gneiss, schist, slate, and granitic rocks range from 15,900 ft. per sec. to 19,200 ft. per sec. (4.8 to 5.8 km./sec.) ; diabase velocities averaged 22,500 ft. per sec. (6.8 km./sec.) ; Triassic sediments from 10,400 to 14,500 ft. per sec. (3.2 to 4.4 km./sec.); and the highest velocity correlated with Cretaceous Coastal Plain sediments is 9,600 ft. per sec. (2.9 km./sec.) The following conclusions are based on these studies. 1) Because basement lithologic trends and velocities are similar to those in the Piedmont, confirmed by deep wells, it is apparent that the Piedmont complex extends under the Coastal Plain sediments as far E. as the present coast. 2) The Carolina Slate belt extends under the Coastal Plain and reaches a maximum width, 80 mi., in North Carolina, though the Triassic deep river basin is in the middle. An eastern slate belt with NE. strike is postulated midway between the Fall Line and the coast under the Coastal Plain sediments. 3) The buried Florence Triassic basin is approximately 40 mi. long on an ENE. strike and limited in width to 13 mi. A new buried Triassic basin is postulated to extend from near Raeford into Johnson County, North Carolina. 4) The Cape Fear arch is a prominent basement structure with a seaward slope of 13 ft. per mi. on the axis. It is not reflected in the sea level basement surface contour near the Fall Line. Sedimentary record suggests differential movement at least twice since Cretaceous time, 5) The pre-Cretaceous basement is a surface of erosion with topographic relief on the order of 200 ft. 6) The break in basement slope in eastern North Carolina must be projected seaward of Cape Fear and the South Carolina coast. 7) There is the suggestion of an E.-W. syncline superimposed on the steeper basement slope in eastern North Carolina, and its existence is supported by a similar structure in the Coastal Plain sediments based on an earlier seismic-reflection survey.