Continental margins are important realms of gas and oil accumulation; 20 per cent of United States production already is offshore. It is necessary, therefore, to try to understand the genesis of continental margins and their subsequent geomorphic evolution. The writer suggests that continental margins are young (generally not older than mid-Mesozoic) and may be classified into two types: (1) accretionary margins caused by the collapse of continental rises and the associated accretion of new fold belts to the continents (Pacific-type); and (2) modified rift scars remaining after continental drift (Atlantic type). Most of the margins around the Atlantic and Indian Oceans are of the latter type but the bulge of Africa and the bight of North America are regarded as Pacific ty e. One can expect the two types to develop somewhat differently insofar as sedimentary modification and geomorphic evolution are concerned. They are either sites of mountain building and hence uplift, or are subsiding isostatically and accumulating a capping wedge of sediments and a thick continental-rise prism. These two elements (the terrace wedge and the rise prism) are "living" examples of the miogeosyncline and the eugeosyncline, respectively. Continental-rise prisms probably are petroliferous, but the petroleum is lost through metamorphism when the rise is collapsed into a eugeosyncline; terrace wedges also are petroliferous, and this petroleum is not lost with time because the presence of sial beneath prevents intense metamorphism. In equating terrace wedges with miogeosynclines, t is to be noted that both have a wedge-shape rather than the form of a syncline. This is regarded as a natural consequence of continental-margin sedimentation. In the search for oil in miogeosynclines, geologists should be aware that they were not closed basins but one-sided "basins" open toward the sea. Thus, instead of basin deposits, one finds mainly those sediments deposited during the prograding and retrograding of paralic-zone sediments.

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