Native bituminous substances are divided into two groups, 1) bitumens and, 2) pyrobitumens. Bitumens are composed principally of hydrocarbons substantially free from oxygenated bodies, are fusible, and are soluble in carbon disulfide. Native bitumens occur in liquid and solid forms. The native liquid bitumens include all petroleums or crude oils. Native solid bitumens include native waxes such as ozocerite, asphalts or petroleum tars, and asphaltites such as gilsonite and grahamite. Pyrobitumens are composed principally of hydrocarbons which may contain oxygenated bodies. They are infusible and are insoluble, or nearly insoluble, in carbon disulfide. Native pyrobitumens are divided into an oxygen-containing group including peats, lignites, and coals, and an essentially oxygen-free, asphaltic group including such substances as wurtzilite, albertite, impsonite, and ingramite. Thucholites, which are carbonaceous substances that may contain uranium, thorium, and rare earths, commonly are considered to be pyrobitumens. Their compositions are variable and may fall into either the oxygen-containing or oxygen-free group. All varieties of native bituminous substances may be associated with mineral matter.

The nomenclature of bitumens and pyrobitumens is used very loosely in the literature. This circumstance arises from the difficulty in recognizing many of these substances by visual examination, and because many of them can be identified accurately only by chemical methods. Inasmuch as some of the chemical procedures are time-consuming and satisfactory analytical methods have not been devised for all these substances, geologists generally have not obtained precise identifications but rather have used names that appeared most appropriate to the circumstances. It is expected that future research will show many substances called "asphaltite," "thucholite," etc., to be incorrectly identified. The nomenclature used by the authors of the various references of this bibliography is followed without deviation or further discussion. The stratigraphic nomenclature also is that used by the authors.

In this bibliography emphasis is placed on reports dealing with the uranium contents and radioactivity of native bituminous substances rather than on mineralogical and chemical studies of these substances. The distribution of the substances described in the references is shown on the accompanying map. The indicated presence of these substances does not infer that they contain sufficient radioactive elements to constitute ores.