Choiseul, 100 × 20 miles, is one of the larger islands of the British Solomon Islands Protectorate, a double chain of islands bordering the Pacific Ocean and separated from the north-east coast of Australia by the Coral Sea. It is an elongated island shaped by faulting and possesses a prominent fracture pattern. Structurally the island is a mass of fault blocks. Movement of these blocks began in early Miocene time and continues at present; reef growth and other coastal features are fault controlled. Stratigraphically the island is an autochthonous system. The geological succession is: basement amphibolite schists; a lava pile over 2,000 feet thick, made up of continental andesites, basalts and basaltic pillow lavas, with minor intrusives; a slab-like body of serpentinous ultrabasics; Lower Miocene grits and biostromal calcarenties about 300 feet thick resting on the lavas; at least 1,000 feet of subgreywackes and volcanic sandstones; Pliocene calcarenites and calcilutites of organic origin, about 2,000 feet thick; Quaternary volcanics given off by two extant volcanic cones, and slabs of uplifted limestone reef masses. Geologically, Choiseul resembles other islands in the group.