CLASSIFICATION OF MINERAL LANDS. 69 



commercially in the United States to-day will yield, after being 

 washed or air dried, at least 8,000 B. t. u. This figure has therefore 

 been fixed as the minimum B. t. u. value of any coal that shall be con- 

 sidered workable. ^ In practice the washed product of any coal sub- 

 ject to washing must have at least 50 per cent of the weight of the 

 unwashed coal. The washability of any coal is determined on a 

 sample of one-quarter inch size by a float and sink test, the liquid 

 used having a specific gravity of 1.5. Even if its workability is thus 

 indicated in the laboratory it can not be considered workable unless 

 there is suiRcient water in the field for washing it. 



THICKNESS. 



More coal is unworkable because it is too thin than for any other 

 reason. The result of a study of the relative cost of mining in 

 relation to the thickness of beds is given on pages 83-86. 



For the purpose of classification a careful study was made of the 

 thicknesses of coals that are actually mined in a large commercial 

 way. Many of the facts disclosed by this study have been published.^ 

 By platting these with reference to the B. t. u. content of the coal it 

 became evident that in actual practice coals yielding 10,000 B. t. u. are 

 worked with profit down to a thickness of 18 or 19 inches, that coals 

 yielding 12,000 B. t. u. are worked down to 14 or 15 inches, and that 

 better coals are mined from beds still thinner. Some coals of the quali- 

 ties mentioned are mined from beds that are thinner than those noted 

 above, but under very exceptional conditions. In considering the low- 

 grade coals three special factors must be noted: (1) Most of them 

 occur in rocks of more recent age than the others, which have been 

 less affected by mountain-making forces and are therefore less indu- 

 rated. On this account it is thought that future extensive mining 

 may show the frequent necessity of leaving some of the coal for 

 roof and of leaving larger pillars. (2) Owing to the air-drying loss 

 in very wet coals they are likely to shrink considerably before they 

 are marketed. (3) If the low B. t. u. value of these coals is due to 

 their high content of ash an allowance should be made for loss in 

 washing. For these reasons the minimum thickness for beds of the 

 low-grade coals is set much higher in proportion to B. t. u. value than 

 for beds of coals of higher grade. 



The minimum minable thiclaiess of bed is fixed at 14 inches for all 

 coals having more than 12,000 B. t. u. For coals having less than 

 12,000 B..t. u. the minimum minable thickness increases at the rate 

 of one-tenth inch for each decrease of 100 B. t. u. down to 11,000, 

 then at the rate of one-tenth inch for each 50 B. t. u. down to 10,500, 

 then at the rate of one-tenth inch for each 25 B. t. u. from 10,500 



1 Fisher, C. A., Depth and minimum thickness of coal beds as limiting factors in valua- 

 tion of coal lands : Bull. U. S. Geol. Surrey No. 424, 1910, pp. 56-62. 



