70 



CLASSIFICATION OB" THE PUBLIC LANDS. 



down to 10,000. The increase for coals yielding less than 10,000 

 B t. u. is at the rate of one-tenth inch for each decrease of 10 B. t. u. 

 in the coal. The following table gives the thicknesses for B. t. u. of 

 even five hundreds : 



Minimum thicknesses tor even five hundreds B. t. u. 



An important question that frequently arises is, "What value, in 

 relation to thickness, shall be given to split or broken beds? The 

 general practice of the United States Geological Survey in classifying 

 coals has been to give a split bed the value of an unbroken bed with 

 which it can fairly be compared. It is evident that a solid 3-foot bed 

 is worth more than two 18-inch benches separated by 6 inches of clay 

 or shale. After careful study the Survey adopted the simple expedi- 

 ent of prescribing that any parting or bench of bone or impure coal 

 included in a bed injured the value of the coal of the bed in amount 

 equal to the thickness of the parting. Thus the split bed just cited, 

 with its 6-inch parting, is regarded as equivalent to a solid bed 30 

 inches thick (36 inches of coal minus 6 inches of parting equals 30 

 inches). If the benches on either side of the parting are not of the 

 same thickness the thickness of the parting is deducted from the 

 thickness of the thinner bench. It is not necessary to consider the 

 whole thickness of a coal bed. It is the practice of the Survey to 

 start with the best bench, if in itself not of workable thickness, and 

 to add the thickness of the next bench above or below after deducting 

 the thickness of the intermediate parting. If the whole bed thus 

 included is still not of workable thickness and more benches exist 

 above or below, the thickness of these benches is added, after sub- 

 tracting the thickness of the parting between them and the principal 

 bench. If a parting is thicker than the thinner adjoining coal 

 bench, that bench is considered as having no value. This practice is 

 best illustrated by a group of concrete examples. (See fig. 2.) 



As a matter of practice two coal layers, each workable, are 

 treated as parts of a single bed if the two layers both exceed in thick- 

 ness the parting between them and the parting does not exceed ^ 

 feet. In general two layers of coal are treated as two beds if the 

 parting between them exceeds either one in thickness, and their work- 

 ability is determined on that basis. 



