Strata of Oligocene and Miocene ages are present on both the Atlantic and Caribbean coasts of Puerto Rico. The strata range in composition from sand and gravel to pure limestone. In northern Puerto Rico seven formations are recognized including four units of coralline and algal limestone. In southern Puerto Rico only two formations are recognized and both are redefined in this paper. The lower formation consists largely of clastic materials, but it locally contains one thick body of reef limestone. The upper formation is here restricted to a thick, very fossiliferous reef limestone.

The strata of middle Tertiary age contain a few folds and minor faults in northern Puerto Rico, but in southern Puerto Rico the strata are cut by many faults, some of which have displacements of several hundred meters.

The reef limestones in both northern and southern Puerto Rico could serve as adequate source and reservoir rocks for petroleum and natural gas. Further study is recommended for several anticlines in northern Puerto Rico and along the faults between Ponce and Guayanilla in southern Puerto Rico that might serve as traps for oil and gas.