halt comes from the famous "pitch lake," which is a
nearly circular deposit covering about a hundred acres 150 feet above
sea level, and which is believed to fill the crater of an old mud
volcano. The so-called pitch consists of a mixture of bitumen, water,
mineral and vegetable matter, the whole inflated with gas, which escapes
to some extent and keeps the mass in a state of constant ebullition. The
surface of the lake is hard, and yet the mass as a whole is plastic and
tends to refill the excavations. The lake is believed to be on the
outcrop of a petroleum-bearing stratum, and the pitch to represent the
unevaporated residue of millions of tons of petroleum which have exuded
from the oil-sands. The pitch is refined by melting,--the heat expelling
the water, the wood and other light impurities rising, and the heavy
mineral matter sinking to the bottom.

The asphalt of Venezuela is similar in nature, but the pitch "lake" is
here covered with vegetation and the soft pitch wells up at certain
points as if from subterranean springs.

FOOTNOTES:

[17] For more detailed treatment of international coal movements before
the war and of coal movements within the United States, see the U. S.
Geological Survey's _World Atlas of Commercial Geology_, Pt. 1, 1921,
pp. 11-16.

[18] Campbell, Marius R., The coal fields of the United States: _Prof.
Paper 100-A, U. S. Geol. Survey_, 1917, pp. 5, 6, 7.

[19] Compiled from tables quoted by White, David, The petroleum
resources of the world: _Annals Am. Acad. Social and Political Sci._,
vol. 89, 1920, pp. 123 and 126.

[20] White, David, _loc. cit._, p. 113.

[21] See Arnold, Ralph, Petroleum resources of the United States: _Econ.
Geol._, vol. 10, 1915, p. 707.

[22] White, David, Late theories regarding the origin of oil: _Bull.
Geol. Soc. Am._, vol. 28, 1917, p. 732.

[23] McCoy, A. W., Notes on principles of oil accumulation: _Jour.
Geol._, vol. 27, 1919, pp. 252-262.

[24] White, David, Genetic problems affecting search for new oil
regions: _Mining and Metallurgy_, _Am. Inst. of Min. Engrs._, No. 158,
Sec. 21, Feb., 1920.

[25] Mehl, M. G., Some factors in the geographic distribution of
petroleum: _Bull. Sci. Lab._, _Denison Univ._, vol. 19, 1919, pp. 55-63.

[26] Schuchert, Charles, Petroliferous provinces: _Bull. 155_, _Am.
Inst. Mining and Metallurgical Engrs._, 1919, pp. 3059-3060.

[27] Loc. cit., p. 20.

[28] Seepages or residual bituminous matter near the surfac