Seismic-reflection profiles over the outer ridge and the Nares Basin north of Puerto Rico show that three principal structures can be traced from the north wall of the Puerto Rico Trench northward to 24° N. between 65°12′ W. and 67°36′ W. The shallowest structure in the Nares Basin consists of flat-lying strata, whereas the corresponding layer on the outer ridge is nearly everywhere acoustically transparent. The transparent layer is thickest near the center of this portion of the outer ridge and thins outward in every direction. The next-deeper structure consists of layers which in places are continuous, but elsewhere are more or less broken up. This series of layers is interrupted by the third structure, a basement having relief of several hundred meters. The basement surface lies about 1 km below the flat floor of the Nares Basin but rises to control the shape of sea floor over parts of the outer ridge. In a few locations where all structures have nearly flat surfaces and also where seismic-reflection data can be compared with older refraction data, the transparent layer corresponds with the 2.1-km/sec layer, the second series of layers corresponds with the 4.2-km/sec layer, and the basement corresponds with the 5.2-km/sec layer. Other possible correlations with refraction data are discussed. The distribution of these three structures suggests a basement high trending irregularly southwest-northeast across the ridge with flanking layered rocks to northwest and southeast. All three structures may have been sampled in the dredging of the north wall in 1962. It is suggested that the transparent layer is a remnant of a continental rise originally extending northward from Puerto Rico prior to the formation of the Trench.

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