A number of seismic-refraction stations were occupied in the eastern Caribbean area, and the results are correlated with other geophysical measurements and with land geology. The purpose of the investigation was to measure the crustal and subcrustal structure in the Caribbean, along the island arc, and in the Atlantic to determine the structural relationship of the interior-basin—island-arc—deep-sea-trench sequence.

The measurements indicate that the Caribbean was formerly an oceanic area which in a large part has been altered to produce a thickened crust and a lowered seismic velocity in the subcrustal material. Along the island arc the crust has been considerably thickened, and its upper surface approximately parallels the topography. On the Atlantic side of the islands the crustal and subcrustal layers are downwarped or downthrust to parallel approximately the topography across the Puerto Rico trench and its extension.

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