n. An oily substance of great economical importance, especially as a source of light, occurring naturally oozing from crevices in rocks, or floating on the surface of water, and also obtained in very large quantity in various parts of the world by boring into the rock; rock-oil.n. Of the entire product of petroleum throughout the world approximately 50 per cent. is furnished by Russia, 40 per cent, by the United States, and 10 per cent. by Canada, Austria, Rumania, the Sunda Islands, Burmah, Japan, and (in quite small proportion) Germany, South America, and Italy. California, Texas, and Kansas have of late largely increased their output, chiefly of crude fuel-oil, while in Pennsylvania, Ohio, and West Virginia there has been a diminished yield. It should be noted that American and Russian petroleum differ materially in chemical composition, the former consisting mainly of hydrocarbons of the paraffin series, while the latter represent chiefly naphthenes, isomeric but not identical with the members of the olefine series.