Petroleum. Crude oil or petroleum is an organic substance consisting of a mixture of hydrocarbons, that is, it is made up very largely of the two chemical elements carbon and hydrogen, in rather complex and variable combinations. It is practically certain that petroleum has been derived by a sort of slow process of distillation from organic matter--animal or vegetable or both--in stratified rocks within the earth. Many strata, as for example carbonaceous shales, are more or less charged with dark-colored decomposing organic matter. The chemical composition itself, the kinds of rocks with which it is associated, and certain optical (microscopic) tests all point to the organic origin of petroleum. In southern California at least, certain of the oils have quite certainly been derived from the very tiny oily plants called diatoms which fill many of the strata. [Illustration: Fig. 77.--Profile and structure section showing folding of strata, with included coal beds, across one of the anthracite coal fields of eastern Pennsylvania. Length of section, a little over 2 miles. (After U. S. Geological Survey.)] During the last twenty years petroleum has come to be one of the most important and useful natural products. Among the many substances artificially derived from petroleum are kerosene, gasoline, naphtha, benzine, vaseline, and paraffine. The United States leads in the production of petroleum, while southern Russia and Mexico are very important producers. In the United States the principal areas underlain with petroleum-bearing strata are the northern Appalachian field (through western Pennsylvania to central West Virginia); the Ohio-Indiana field (central Indiana to northwestern Ohio); the mid-continental field (southeastern Kansas and northeastern Oklahoma); the southeastern Texas-Louisiana field; and the southwestern California field. The total areas underlain with oil total about 10,000 square miles. In the Appalachian, Ohio-Indiana, and mid-continental fields the strata carrying oil range in age from Ordovician to Pennsylvanian, and they are mostly but little disturbed from their original horizontal position. The Texas-Louisiana oils come mainly from Cretaceous and Tertiary strata which gently downtilt under the Coastal Plain toward the Gulf. In California the oil-bearing strata are of Tertiary Age and generally considerably disturbed and folded.