The great economic importance of these substances requires that brief mention of them be made, though they can hardly be considered abundant as rocks. The natural hydrocarbons of the earth's crust belong principally to the methane series, with the general formula CnH2n+2. The most abundant are marsh gas (CH1), petroleum, a mixture of several hydrocarbons, which are liquid at ordinary temperatures, and asphalt, which is solid or extremely viscous, and results from the oxidation of hydrocarbons. The hydrocarbons impregnate porous or shattered rocks, which they have invaded from below, and are frequently retained under great pressure by overlying impervious beds. Natural gas and petroleum tend to collect in the upward arches (anticlines) of folded beds, and when these reservoirs are tapped by the drill, the oil and gas rise in spouting wells which may continue to flow for many years.

While certain eminent chemists have maintained the inorganic origin of the hydrocarbons, there is no evidence that they actually were formed in this way, and nearly all geologists are agreed that they have been derived from the fatty and oily parts of organic accumulations, both animal and vegetable, at high temperatures and pressures. That such a mode of generation is at least possible has been demonstrated experimentally, and the geological mode of occurrence of these hydrocarbons renders the hypothesis of their derivation from organic substances extremely probable. Petroleum is found in rocks of a very wide range in geological time, and the various oil-fields of the United States are of very different geological dates.

Asphalt is found in beds interstratified with ordinary sediments or in cavities and fissures of the rocks, or impregnating porous limestones and sandstones.