Seismic-reflection measurements in the smoother parts of the ocean-basin floor fall into two general groups. Reflection records from the oceanic rises generally show a succession of reflections which can be correlated for considerable distances. Reflection records from the abyssal plains in general show many reflections which are usually impossible to correlate even between adjacent shots. This difference has been explained by Ericson, Ewing, and Heezen (1952) in terms of the distribution of turbidity-current deposits. In the abyssal plains relatively frequent turbidity flows have deposited an alternating sequence of clays and silts which return a great number of reflections to the reflection seismograph. In contrast the rises receive only pelagic sedimentation, and thus the layering of their sedimentary cover is simple and widespread since it relates to major changes in pelagic sedimentation of past geologic ages. [Illustration: FIGURE 35.--_Crustal sections in various physiographic provinces, determined by seismic-refraction measurements_ (a). Eastern New York; Katz and Ewing (1955) (b). Western Sohm Abyssal Plain; Station A 172-28, from Katz and Ewing (1955) (c). Bermuda Apron; Station A 172-20 from Katz and Ewing (1955) and Officer _et al._ (1952) (d). Bermuda Plateau; Station A 172-19, from Katz and Ewing (1955) (e). Nares Abyssal Plain; personal communication from J. I. Ewing (f). Mid-Atlantic Ridge; personal communication from J. I. Ewing] MID-OCEANIC RIDGE DEFINITION The third basic subdivision of the oceanic depression is the Mid-Oceanic Ridge, a continuous median ridge which runs the length of the North Atlantic, South Atlantic, Indian and South Pacific oceans, for more than 40,000 miles (Heezen and Ewing, in press). In the center third of the physiographic diagram a short segment of this world-encircling ridge is represented. MID-ATLANTIC RIDGE One can find references to the Mid-Atlantic Ridge in the scientific literature dating back more than 80 years. Before the advent of the echo sounder the lateral limits of the Mid-Atlantic Ridge were very difficult to define, and even now widely different definitions are used. Murray (1912) mentioned that the ridge lay in depths less than 2000 fathoms but pointed out that locally on the ridge depths exceeded 2000 fathoms. The METEOR expedition charts and profiles generally imply by their labeling that the ridge is the area enclosed by the 4000-meter contour (2250 fathoms). Shepard (1948) states that its "average depth is about 1500 fathoms, but it rises about 1000 fathoms above deeper zones on either side."