Tentative matching of geological provinces of the same age shows how South America and Africa presumably fitted together some 200 million years ago. Dark areas, ancient continental blocks, called cratons, that are at least 2,000 million years old; light areas, younger zones of geological activity-mostly troughs filled with sediments and volcanic rocks that were folded, compressed, and intruded by hot materials, forming granites and other rock bodies. Much of this activity was 450 million to 650 million years ago, but some of it goes back 1,100 years. Dots show the sites of rocks dated by many laboratories, including the author's at the Massachusetts Institute of Technology: solid dots, rocks older than 2,000 million years; open dots, younger rocks. The region near São Luis is part of an African craton left stranded on the coast of Brazil (Hurley, 1968).
