The ocean-basin floor provinces are virtually devoid of earthquakes of a size detectable at distant seismic observatories. Of course small earthquakes (< 5, Richter scale) would probably not be locatable in such remote regions. The virtual absence of larger shocks makes it improbable that many small ones occur there either. Two earthquakes have occurred in the Bermuda Rise, one near the west boundary and one in the scarp zone of the southeast Bermuda Rise (Pl. 29). Several quakes were felt in Bermuda before instrumental recording was available to permit location of their epicenters. Two quakes occurred in the abyssal plain northwest of the Cape Verde Islands, and two were located near Theta Gap northwest of Cape Finistere. All other earthquakes of the central part of the ocean basin are associated with the Mid-Atlantic Ridge or its eastern extension. OCEAN-BASIN FLOOR PROVINCES AND CRUSTAL STRUCTURE The results of seismic-refraction measurements in the ocean-basin floor can be divided into two categories depending on whether the measurements were made (1) in the abyssal floor, or (2) on an oceanic rise. Measurements in the abyssal floor of the western Atlantic (Ewing, Sutton, and Officer, 1954) revealed the simple pattern shown in Figure 35_b_ and _e_--namely, that beneath 4-5 km of water lies .5-1 km of sediments and sedimentary rock with a compressional-wave velocity of about 2 km/sec, overlying 3-4 km of oceanic crustal rocks (6.5 km/sec); beneath this the sub-M mantle rocks show a velocity of about 8.1 km/sec. This pattern has been observed by most workers in the abyssal floors of other oceans (Raitt, 1957; Hill, 1956). Officer, Ewing, and Wuenschel (1952) and Katz and Ewing (1955) have reported on the structure of the Bermuda Rise. The topographic change from abyssal floor to the Bermuda Rise is accompanied by a corresponding change in crustal structure (Fig. 35_b_ and _d_). A typical column measured on the Bermuda Plateau is shown in Figure 35_d_. Here the sub-M velocity appears to be lower, or possibly a new intermediate-velocity layer is inserted between the oceanic crust and the true mantle. In the Bermuda Rise seismic velocities in the oceanic crust differ somewhat from the typical abyssal-floor values. In the Bermuda Apron and adjacent parts of the Bermuda Plateau (Fig. 35_c_) above the oceanic crust a thick (up to 4 km) section of 4.5 km/sec velocity is found which has been quite reasonably identified as sedimentary and volcanic rocks.