Three dredge hauls from the north wall of the Puerto Rico Trench near 66° 30′ W. contained serpentinized peridotite and talc rock, sedimentary rocks of Cretaceous and Tertiary age, and minor basalt. A possible stratigraphy of the main scarp is given by serpentinized peridotite with altered basalt overlain by Upper Cretaceous sedimentary rocks, which are in turn overlain by Lower Tertiary sedimentary rocks with minor vitric basalt. The ultramafic rocks occur as two distinct types. One contains antigorite and talc and has a rare texture consisting of very fine-grained olivine(?) replaced by talc or serpentine. The other does not contain antigorite or talc and has a texture similar to serpentinites from many localities in the world. The density, seismic velocity, magnetic susceptibility, and chemical data of the ultramafic rocks are presented. Structural complexities of the area are revealed by two closely spaced seismic-reflection profiles of the main scarp of the north wall, one showing horizontal crustal layers and the other an interrupting mass. The 5.1 km/sec seismic layer determined by previous seismic-refraction studies may here be composed primarily of serpentinized peridotite with some altered basalt. Navigational uncertainties and lack of knowledge of the exact position of dredging prevent definite statements. However, the 5.1 km/sec seismic layer was definitely sampled about 65 km to the east (20°16′N., 65°43′W.), and basalts, chert, and Cenomanian siliceous sedimentary rock were obtained. These rocks may overlie serpentinized peridotite that was not sampled, or possibly serpentinite does not occur on the north wall near 65°45′W.

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