Probable changes in practice that may be expected to affect the petroleum industry within the next ten years include an increased dependence by oil producers on geologic investigations in advance of drilling, the development of methods for deeper drilling than is now practicable, and the more efficient handling of individual wells and of entire properties, with a view to the ultimate recovery, at minimum cost, of a higher percentage of the oil originally present. The tendency toward amalgamation of individual producing, transporting, refining and marketing interests into strong units capable of competition in domestic and foreign markets on relatively equal terms with each other and with pre-existing combinations of equivalent strength will doubtless increase, and with the growing strength of the several units will come an efficient and thorough quest for petroleum in all parts of the world. In the refining of petroleum it is probable that methods will be devised and perfected for recovering more of the light-gravity products from low-grade petroleum and for the conversion of the less-salable products of petroleum into products of greatest current demand. Moreover, it is believed that internal-combustion engines will be so modified as to run successfully on petroleum products of lower volatility than gasoline. The use of petroleum as railroad, marine, and industrial fuel is destined to increase enormously in the next decade. Although an important contributor to the oil-supply of Great Britain, the shale-oil industry has received little attention in recent years outside of Scotland. Investigations by the United States Geological Survey have demonstrated that the United States contains vast deposits of oil shale in Utah, Colorado, Wyoming and Nevada, much of which will average higher in oil content than the Scottish shale. Efforts already begun to develop methods for the recovery of shale oil on a commercial scale in the United States will undoubtedly result in the establishment of a shale-oil industry in this country within the next two or three years. The future growth of this industry will depend largely on the rapidity of the decline in the domestic production of petroleum. GEOLOGICAL DISTRIBUTION Commercial accumulations of petroleum are everywhere restricted to strata of sedimentary origin. In the United States petroleum is produced commercially from strata of all periods from Cambrian to Quaternary, the most prolific sources being in strata of the Carboniferous and Tertiary systems. The geological age of the chief sources of petroleum production in each of the other oil-producing countries of the world is indicated in the table following: TABLE 1.--GEOLOGIC AGE OF PETROLEUM-BEARING FORMATIONS