The geology of the past controls the distribution of the mineral deposits of the present. Veins and lodes of metals have been found only in association with rocks of continental origin, for example, granite and various metamorphic rocks that have been subjected to great heat and pressure and especially to the work of hot solutions. Recent lavas rarely contain workable deposits of metallic minerals. The continent of Australia, the islands of Indonesia, the Philippines, New Guinea and New Zealand, and the supposed fragments of a former continent, like New Caledonia, some of the Louisiade Archipelago, and part of the Fiji Islands possess metallic deposits. Coal and oil are associated only with sedimentary rocks. Excluding Japan and the Asiatic mainland, the coal so far mined has come principally from Australia and New Zealand, and the petroleum from Borneo, Java, and Sumatra.
The only valuable mineral sometimes found on coral islands is phosphate rock. Deposits of guano have been exploited on numerous low islands that were the breeding grounds for sea birds and where the light rainfall was insufficient to wash away the manure from which guano is formed. The phosphate rock is supposed to have been formed by the interaction between the lime of the coral bedrock and phosphoric acid leached from the guano. Commercial deposits of phosphate rock are worked extensively on Nauru, Ocean, and Christmas islands, all near the equator, which supply one-tenth of the world's demand for phosphate rock. Angaur Island in the Palaus, several islands in the western Carolines, and Makatea, in the Tuamotus north of the Society Islands, also mine phosphate rock.
Guano has come from scores of islands but is now largely exhausted in the mid-Pacific area. Under an old law, ship captains of the United States could claim sovereignty for their country over any unclaimed island valuable for guano. This law permitted a discoverer to mine guano with legal protection from others. Under this law about 70 islands were claimed for the United States between 1858 and 1880 and many were occupied for a time. Although claims to many lapsed through non-occupancy, title to Howland, Baker, Johnston, and Jarvis were secured in this way. Other nations had laws similar to our guano act, and some islands had several claimants, each of whom might put a different name on the map, which naturally led to confusion.