For at least 300 years prior to the beginning of the 20th century, people had noticed that the bulge along the eastern edge of South America fits remarkably well into the right of Africa. Indeed, Francis Bacon suggested in 1620 that the fit could not be accidental. Several 19th century scientists offered explanations for the fit, but when Alfred Wegener first published the continental-drift hypothesis in 1912, his novel explanation for why the continents seem to fit together like pieces of a jigsaw puzzle drew explosive criticism from many geologists. Wegener used gravity measurements and observations of the Earth's surfaces to deduce that the continents are composed of lighter rock than the basalt that lies beneath the ocean floors. The continents, he suggested, float on the denser layer of basalt like icebergs on water. Wegener's early critics excoriated him for not proposing a mechanism for propelling the continental “ice bergs” through solid basalt, although now there is some theoretical basis for thinking that convection might drive the process. Wegener did not live long enough to find the clear and convincing evidence his hypothesis required, but since his death in 1930, geologists have learned much that supports his revolutionary idea. For example, belts of complementary rock formations found along the African and South American shorelines of the Atlantic Ocean make a strong argument for continental drift. In one experiment, geochronologists determined the age of a distinctive layer of crystalline basement rock in Ghana, Africa, and then predicted where the same rock layer would be found at the edge of South America if the two continents had indeed once been contiguous. By sampling and dating rocks in northeastern Brazil, the scientists demonstrated that the layer does occur in its predicted location. Another belt of two-billion-year-old rock that abruptly ends at the edge of the West African continental mass and begins again at the expected location along the coast of South America adds additional proof that the two continents once formed part of a larger land mass. The fossil record contains additional evidence that Africa and South America were once connected. Large bodies of water act as barriers to the migration of many types of animals, yet the fossils of identical animal species are found on both sides of the southern Atlantic Ocean. For example, the remains of Mesosaurus, a small reptile of the Permian that lived in shallow, brackish swamps, are found in only two locations-in the Early Permian Dwyka Formation in South Africa and in the Irarare Formation in Brazil. The rock formations are the same age, are similar in composition, and lie directly across the ocean from each other, thus enhancing the plausibility of Wegener's theory of continental drift.