=Inorganic theory of origin.= Another theory of the source of oil has had some supporters, although they are much in the minority. This is the so-called "inorganic" theory, that oil comes from magmas and volcanic exhalations. In support of this theory attention is called to the fact that igneous rocks and the gases associated with them frequently carry carbides or hydrocarbons; that many oil fields have a suggestive geographic relationship with volcanic rocks; and that certain of the oil domes, as for instance in Mexico, are caused by plugs of igneous rocks from below. It has been suggested that deep within the earth carbon is combined with iron in the form of an iron carbide, and that from the iron carbide are generated the hydrocarbons of the oil, either by or without the agency of water. Iron carbide is magnetic, and significance has been attached to the general correspondence between the locations of oil in the western United States and regions of magnetic disturbance. It seems not unlikely that some inorganic theory of this sort is necessary to explain the ultimate source of oil or of the substances which become oil, but the evidence is overwhelming that organic agencies have been mainly responsible for the principal oil pools now known. =Oil exploration.= A simple geographic basis for oil exploration is the fact that the major oil fields of the world are situated between 20 deg. and 50 deg. north latitude, and that thus far there are no major oil areas within the tropics or within the southern hemisphere. This broad generalization may have little value when exploration is carried further. It has also been suggested that the geographic distribution of oil corresponds roughly with the average annual temperatures, or isotherms, between 40 deg. and 70. deg.[25] It is thought that this present distribution of temperatures may indicate roughly the temperatures of the past when the oil was accumulated; and the inference is drawn that there was some sort of limitation of areal deposition within these temperature limits. If this be true, the only reasons why the southern hemisphere is not productive are the relatively small size of the land areas and the lack of exploration to date. In approaching broadly the problem of oil exploration, the geologist considers in a general way the kinds and conditions of rocks which are likely to be petroliferous or non-petroliferous. Schuchert[26] summarizes these conditions for North America as follows: 1. The impossible areas for petroliferous rocks. (_a_) The more extensive areas of igneous rocks and especially those of the ancient shields; exception, the smaller dikes. (_b_) All pre-Cambrian strata. (_c_) All decidedly folded mountainous tracts older than the Cretaceous; exceptions, domed and block-faulted mountains. (_d_) All regionally metamorphosed strata.