Puerto Rico, the easternmost and smallest of the Greater Antilles, has an axis of deformed and metamorphosed volcanic rocks of Upper Cretaceous age, intruded by dioritic rocks during the Antillean revolution. The hard-rock core is flanked on the north and south by limestones and clastic rocks of late Oligocene and early Miocene age, which have been gently arched and uplifted. Similar rocks were deposited in late Miocene or early Pliocene time along the west coast. During the Quaternary the island has been separated from the other major Antillean islands by faulting and has been arched, uplifted, and tilted to the northeast. Alluvium and littoral deposits have partially filled the valleys and have formed coastal plains on the north and south. The Upper Cretaceous volcanic and associated rocks yield small supplies of water to wells in most places. The Tertiary limestones yield large supplies in some places on the north, south, and west coasts. The Quaternary sands and gravels are the most important aquifers. They yield about 200 million gallons a day to wells in the main south coastal plain alone. The water is used largely for irrigation. A total of perhaps 250 to 300 million gallons a day is pumped from wells in the island, and moderate to large additional supplies are available in some places. © 1947 Society of Economic Geologists, Inc.