ABOUT 150 m.y. ago, a successful rift formed which led to the separation of present day South America from Africa. Many types of rocks from Precambrian migmatites to Lower Cretaceous evaporites1 match across the junction of rifting; but why do continental masses rift where they do? The answer could lie in purely random phenomena, but time after time ancient trends and structures seem to control later events. If events in the mantle are random, perhaps they lead to successful amplification only when they interact with certain structural situations in the continental crust. Only recently have sufficient data become available on the basement rocks of the Atlantic coasts of Africa and Brazil to allow a preliminary synthesis. But it seems certain that the rifting followed an ancient intercratonic structural trend (mobile belt) with a complex history of metamorphism and magmatism. The rifting process was not random.