The most prolific sources of petroleum are in sedimentary strata of the Carboniferous and Tertiary periods. Because the most detailed geologic work is insufficient to provide for the appropriate evaluation of the numerous factors involved in the occurrence of petroleum, and because only a relatively small percentage of the areas of sedimentary rocks throughout the world have been examined geologically in any appreciable detail, it is difficult to estimate the future supply of petroleum or to predict that large accumulations will be discovered in any particular region. The principal countries contributing to the world’s production of petroleum rank as follows in general order of importance: United States, Russia, Mexico, Dutch East Indies, Roumania, India, Persia and Galicia. Other countries produce less than 2 per cent. of the annual total. The greatest change that is likely to come in the geographical distribution of production is a larger output from the countries bordering the Caribbean Sea and the Gulf of Mexico, and from the Persian and Mesopotamian fields. Mexico now ranks second to the United States, and South American countries promise to become more important contributors to the world’s production than they now are. Russia is expected to become ultimately one of the chief producers of petroleum. Within the next decade, through improved methods of production and through the further amalgamation of producing, transporting, refining, and marketing companies into strong units, the output will undoubtedly be larger and will be more economically produced. In the refining of petroleum it is probable that improved methods will make possible the recovery of a larger percentage of lighter products from low-grade petroleum. Internal-combustion engines are being modified so as to run on petroleum products of lower volatility than gasoline. The use of petroleum as fuel under railroad and marine boilers is expected to increase enormously in the next decade. As the output of the producing fields declines, the vast deposits of oil shale in the western United States will be developed as a source of oil. So far as is known, political control of the petroleum resources of the world is determined by state sovereignty (see Plate I, page 7). In normal times, the United States controls politically over 66 per cent. of the present output of petroleum. Russia and Mexico ranked second and third in 1917, controlling 13.6 per cent. and 10.9 per cent., respectively. The remaining 9 per cent. was controlled by Great Britain, Holland, Persia (British government owned), Roumania, Austria-Hungary, Japan, Peru, Germany, Argentina and Italy in the order named.