CLASSIFICATION OF MINERAL LANDS. 83 



VAIiTJB OP THUT COAI/S. 



The cost of mining thin coals increases rapidly with the decrease 

 in their thickness, for the following reasons : The rate paid for min- 

 ing the coal increases with the thinning of the bed, as do the amount 

 of dead work per unit of output, the cost of trackage, ties, rails, haul- 

 age, and ventilation, and, for the very thin coals, the cost of taking 

 up floor or brushing down roof to obtain height. 



H. M. Chance^ has prepared curves expressing the relations 

 between mining cost and thickness for anthracite coal. The figures 

 showing Chance's determinations are given in column 2 of the table 

 on page 84. Similar figures (column 3) were obtained by the Survey 

 for bituminous coals. Detailed cost sheets of mines working coals of 

 different thicknesses were -taken and the costs were separated into 

 four items— (a) capital cost (interest, etc.); (&) mining rate; (c) 

 tonnage cost (proportional to output, tipple, office, etc.) ; (d) acreage 

 cost (proportional to acreage mined out, dead work, hauling, etc.). 

 Obviously, if the recovery per acre-foot is the same, the last item 

 would be double for a 5-foot coal bed what it would be for a 10-foot 

 bed yielding the same output and would be inversely proportional 

 to the thickness of the bed. Though the tonnage recovered is greater 

 per acre-foot from thin than from thick beds, yet the recovery 

 varies so greatly from mine to mine and from time to time that an 

 absolutely invariable recovery from a bed of a given thickness can 

 not be assumed. Therefore, in reaching the results now to be stated, 

 differences in recovery from beds of different thickness have not 

 been considered. As in all studies of coal mining made by the 

 Geological Survey the obtainable figures relating to any particular 

 factor were, if possible, gathered together and platted graphically on 

 a chart, by which it has usually been practicable to show certain 

 definite averages or curves expressing the relation sought. Whenever 

 possible, these curves have been reduced to mathematical formulas. 

 A concrete example is a curve showing the relation between the min- 

 ing rate and thickness of bed, a relation which it was necessary to de- 

 termine in working out the total cost of mining beds of different 

 thicknesses. In making this diagram the mining rates at different 

 places in the United States were platted on cross-section paper as 

 shown in figure 4 (p. 85), on which the curve A-B was drawn to ex- 

 press the average relation between these two factors, a relation which 

 was used in later computations. 



1 Chance, H. M., The cost of mining coal: Eng. and Min. Jour., May 29, 1909, pp. 

 1099-1101. 



