bituminous rock for domestic
consumption. Deposits of natural asphaltic material are widely
distributed through the United States, but commercial production is
limited to a few localities in Kentucky, Texas, Utah, Colorado,
Oklahoma, and California.

The asphalt manufactured from petroleum constitutes a much larger
tonnage than natural asphalt though it does not enter so largely into
world trade. The manufactured product is largely but not exclusively in
American control. Large amounts are made in this country and will no
doubt be made for the next decade, from oil produced in the southwestern
states and in Mexico. At the present time as much or more asphalt is
made in the United States from Mexican as from domestic crude oil. The
refineries are located near the Gulf coast so that exports can avoid
overland shipments. The relative merits of natural asphalt and asphalt
manufactured from oil may be subject to some discussion; but it is
perfectly clear that the manufactured material is sufficient, both in
quantity and variety, to make the United States entirely independent and
have an exportable surplus.

GEOLOGIC FEATURES

Natural asphalt and similar products are in the main merely the
residuals of oil and gas distillation accumulated by nature under
certain conditions already described in connection with oil (pp.
140-144). In some cases the asphaltic material is found as impregnations
of sediments, and appears to have remained in place while the lighter
organic materials were volatilized and migrated upward. In other cases
it occurs in distinct fissure veins; the fissures and cavities
apparently were once filled with liquid petroleum, which has
subsequently undergone further distillation. The original liquid
character of some of these bitumens is shown by occasional fragments of
unworn "country rock" imbedded in the veins. The effect of surface
waters, carrying oxidizing materials and sulphuric acid, is believed to
have contributed to the drying out and hardening of these veins or
dikes.

Asphalts and bitumens include a wide variety of hydrocarbon materials,
such as gilsonite, grahamite, elaterite, ozokerite, etc., which are used
for somewhat different purposes. The deposits of the United States show
much variety in form, composition, age, and geologic associations. The
important Kentucky deposits occur as impregnations of Carboniferous
sandstones at the base of the Coal Measures of that state.

The Trinidad asp