Interest in the quantity and quality of the coal reserves of the United States has increased greatly since the end of World War II, principally because of the growing realization that the ultimate reserves of petroleum and natural gas, although largely undefined, still. have finite limits. With the greatly increased use of petroleum and natural gas, it has become further apparent that the reserves of these two fuels, whatever their ultimate limits may prove to be, are being consumed at a rate far surpassing that anticipated a few years ago. At some time in the future, therefore, the contribution of coal to the total production of energy in this country must inevitably be enlarged to include some of the needs now served by petroleum and natural gas.

Although coal-bearing rocks cover 14 percent of the total area of the United States (fig. 1) and contain enormous reserves, it is equally apparent that reserves of coal also have limits. In the extensively mined sections in the East it is already increasingly difficult to locate new areas containing thick beds of high-rank and high-quality coal to replace areas that have been mined out. Furthermore, a considerable part of the total reserves of the United States consists of coal of lignite and subbituminous ranks and coal contained in thin beds that can be mined only with great difficulty and expense. At the present time, therefore, the depletion of reserves of high-rank and high-quality coal, particularly the Eastern coal that is suitable for the manufacture of metallurgical coke, is a more serious problem than the percentage depletion of the total coal reserves.

Recognizing the need for more detailed estimates of coal reserves than those that have been available in the past, the U. S. Geological Survey is now preparing a reappraisal of the coal reserves of the United States in which primary emphasis is placed on the amounts of coal in separate categories according to rank,thickness of coal, and thickness of overburden. Many of the state geological surveys in coal-producing areas are also preparing new appraisals of coal reserves. The increasing volume of geologic data available on the occurrence of coal and the detailed and careful methods now employed in calculating reserves should ultimately provide a more reliable estimate for the coal reserves of the United States than has been obtainable previously, although much additional work remains to be done.