A legend in Trinidad has it that God created the islands of the Caribbean by shaking loose from his fingers a fistful of earth and letting it fall into the waters below. Geologists are less fanciful. Of the many thousands of islands in the 1.5-million- square-mile Caribbean Sea, almost all are the summits of par- tially submerged volcanoes strung like vertebrae in a gentle arc from Florida to Venezuela. The vegetation is lush, the soil fertile enough in places for a few crops, the rainfall abundant-30...