The ocean-basin floor lies between the continental margin and the Mid-Oceanic Ridge and consists of the deeper abyssal floor and the elevated oceanic rises. On the abyssal floor adjacent to the continental margins are found the flattest surfaces of the earth. These abyssal plains apparently were built by turbidity-current deposits. The unburied abyssal floor is represented by the abyssal hills. The oceanic rises are broad uplifts which rise from the abyssal floor through a series of scarps. Oceanic rises are covered with pelagic sediments except locally near islands and seamounts. The crustal structure of oceanic rises differs significantly from the typical abyssal floor in having lower velocities and generally thicker crustal layers. The Mid-Oceanic Ridge is a broad fractured arch whose axis follows the median line of the ocean. It generally covers the center third of the ocean. The ridge provinces are divided into crest provinces and flank provinces. The crest provinces include (1) the Rift Valley, a long axial cleft 15-30 miles wide and 500-1500 fathoms deep; (2) the Rift Mountains which form the sides of the Rift Valley; and (3) the High Fractured Plateau, a rugged plateau which borders the Rift Mountains. The flank provinces consist, on each side, of roughly three steps separated by large scarps. A seismic belt accurately follows the Rift Valley. The topography of the Mid-Oceanic Ridge seems best explained by extensive normal faulting. The mid-oceanic Rift Valley connects with and is probably of the same origin as the African rift valleys. BIBLIOGRAPHY AGOSTINHO, J., 1937, Volcanic activity in the Azores: Bulletin Volcanologique, v. II (Series II), p. 380-400 ANDRADE, C. F. DE, 1937, Os vales Submarinos Portugueses: Servicos Geologicos de Portugal, Lisboa, 235 p. ANONYMOUS, 1935, General bathymetric chart of the oceans, Sheet A1: Information concerning the preparation of the third edition of Sheet A1; North Atlantic Ocean: Int. Hydro. Bur., Spec. Pub. 30, 30 p. ---- 1939, A summary of echo sounding apparatus: Int. Hydro. Bur., Monaco, Spec. Pub. 33, 137 p. ---- 1953, Limits of oceans and seas: Intl. Hydro. Bur., Monaco, Spec. Pub. 23, 23 p. ARMSTRONG, J. C., 1953, Oceanography in the Tongue of the Ocean, Bahamas, B.W.I.: Special Report of the American Museum of Natural History, N. Y., 63 p. ATWOOD, W. W., 1940, The physiographic provinces of North America: Ginn and Co., Boston, 536 p. BARTHOLOMEW, J. C. & SON LTD., 1952, Map of Western Europe, scale 1:3,000,000: Edinburgh Geographical Inst., Edinburgh, Scotland ---- 1955, Map of Africa, scale 1:10,000,000: Edinburgh Geographical Inst., Edinburgh, Scotland