COAL 



139 



percentage of coal classified as measured and indi- 

 cated will surely increase. 



Thickness of beds. — Coal in thick beds, 0-1,000 

 feet below the surface comprises 4 percent measured, 

 8 percent indicated, and 13 percent inferred, for a 

 total of 25 percent of the identified resources shown 

 in figure 16. This percentage, when applied to the 

 total of 1,581 billion tons, is equivalent to nearly 

 400 billion tons. This choice tonnage is in a thick- 

 ness and overburden category comparable to that of 

 coal now being mined, and is therefore of current 

 and near-current economic interest. 



Coal in beds of intermediate thickness, 0-1,000 

 feet below the surface, makes up 23 percent of the 

 identified resources, and is equivalent to 350 billion 

 tons. This tonnage is of less immediate economic 

 interest than tonnage in the thicker beds. However, 

 some coal in this thickness and overburden category 

 is currently being mined, and the total must be 

 considered a paramarginal resource that will be- 

 come of increasing economic interest and importance 

 in the future. 



Coal in thin beds, 0-1,000 feet below the surface, 

 makes up 41 percent of the identified resources, and 

 coal in all thickness categories, 1,000-3,000 feet 

 below the surface, makes up the remaining 11 per- 

 cent. This coal is of little current economic interest. 



The amount in any category or combination of 

 categories can be derived from figure 16 by the 

 procedure used above. 



STRIPPING COAL RESOURCES 



In a recently published study, the U.S. Bureau of 

 Mines (1971) concluded that the remaining strip- 

 ping coal resources of the United States as of Janu- 

 ary 1, 1968, totaled 118 billion tons. Of this total, 

 about 90 billion tons, or 80 percent, is within reach 

 by present machinery and methods of mining, but 

 only 45 billion tons is both available for use and 

 economically recoverable. 



For purpose of comparison, the larger total of 

 118 billion tons of stripping coal resources is 7.5 

 percent of the total of 1,581 billion tons of remain- 

 ing identified resources as reported in table 27. 



The 45 billion tons of potentially recoverable 

 stripping coal includes 32 billion tons of low-sulfur 

 coal (less than 1 percent), 4 billion tons of medium- 

 sulfur coal (1-2 percent), and 9 billion tons of 

 high-sulfur coal (more than 2 percent). 



HYPOTHETICAL RESOURCES 



The preceding analysis of the distribution of 

 identified coal resources provides convincing evi- 

 dence that unmapped and unexplored areas in 



known coal fields contain substantial additional re- 

 sources that must be classed as hypothetical. The 

 approximate magnitude of the additional hypo- 

 thetical resources has been estimated by a process 

 of extrapolation from nearby areas of identified 

 resources, and estimates for each State are pre- 

 sented in separate columns in table 27. The total 

 tonnage of hypothetical resources actually exceeds 

 by a small amount the tonnage of identified re- 

 sources. Figure 17 shows the percentage relation 

 between identified and hypothetical resources in 

 four overburden categories. 



EXPLANATION 



MM 



Identified resources 



Hypothetical resour< 



Figure 17. — Probable distribution of total estimated U.S. 

 coal resources according to thickness of overburden. 



Although large, the hypothetical resources are, 

 for the most part, relatively inaccessible for mining 

 at present, and a more exact delineation of the 

 magnitude, distribution, and future utihty of such 

 resources will require a substantial amount of de- 

 tailed geologic mapping, exploration, and study over 

 a long period. Nevertheless, the estimated hypo- 

 thetical resources constitute an important part of 

 the total resource that needs to be considered in 



