86 CLASSIFICATION OF THE PUBLIC LANDS. 



ing to the formula given on page 84, it was decided to avoid a 

 sliding scale both in value and recovery by reducing the value of beds 

 less than 6 feet thick 10 per cent for each foot below 6 until the 

 minimum for that coal is reached. Thus the coal in a 5-foot 

 bed is computed at 90 per cent of the value per ton of the coal 

 in a 6-foot bed, the coal in a 3-foot bed at 70 per cent of the value 

 per ton of the coal in a 6-foot bed, and so on. This reduction for the 

 value of the coal in beds less than 6 feet thick is readily computed by 



multiplying the normal value per ton by ^j^, where t is the thickness 



in feet. Figures showing the results obtained by multiplying the 

 relative value of coal in thin beds, given in column 8 of the preceding 

 table, by various assumed rates of recovery were plotted on cross- 

 section paper and an arbitrary line was drawn through the middle 

 of the group of lines so obtained. This line is practically a straight 

 line, giving a reduction in value' of 10 per cent for each 1-foot reduc- 

 tion in thickness, and was accepted as representing fair average 

 relative values. 



Sufficient data were not at hand on the relative cost of mining coals 

 more than 10 feet thick to lead to definite figures, or figures having 

 more than approximately authoritative value. It is, however, gen- 

 erally recognized that the cost of working coals does not continue to 

 decrease indefinitely as the beds increase in thickness, for the in- 

 creased cost of timbering and increases due to other practical diffi- 

 culties gradually overcome the gain due to smaller acreage, and in- 

 creases for the thick coals in general probably entirely offset that 

 gain. From beds more than 10 feet thick, other things being equal, 

 the recovery per acre-foot decreases with increase in thickness, owing 

 to the necessity of leaving larger pillars and the difficulty of reaching 

 all the coal. It has therefore been assumed that the recovery dimin- 

 ishes on each additional foot above 10. Thus a 16-foot bed takes first 

 the normal value on 10 feet ; next the eleventh foot is valued at only 

 99 per cent of the value fixed for the tenth foot, the twelfth foot at 

 98 per cent, and so on, the sixteenth foot having a value of only 94 

 per cent of the value fixed for the tenth foot. 



In computing acre values on beds over 10 feet thick it has been 

 found best to use the normal basing value in cents per ton of a 10- 

 foot bed and assume the same recovery per acre-foot and then com- 

 pute the thickness (f) of a bed that wi£hout deduction will yield 

 the same acreage recovery as the bed in question. Where the thick- 

 ness in feet (t) is more than 10 the equivalent thickness {t') is ex- 

 pressed by the following formula: f = 9+ -^-(^ TTvr)- 



