CLASSIFICATION OF MINERAL LANDS. 



75 



feet thick; but, especially in the West, the cost of mining coals less 

 than 6 feet thick increases as the thickness decreases. It has there- 

 fore been assumed that a coal of the minimum thickness for its grade 

 is workable to a depth of not more than 500 feet ; that a coal 6 feet 

 or more thick is workable to a depth of 100 feet for each 300 B. t. u. it 

 contains ; and that a coal between its minimum thickness and 6 feet is 

 \Yorkable to a depth between 500 feet and the maximum depth limit 

 for that coal proportional to the thickness above the minimum. 

 Thus, a 12,000 B. t. u. coal 6 feet or more thick has a maximum depth 

 limit of 4,000 feet and an assumed minimum thickness of 14 inches ; 

 a bed of 12,000 B. t. u. coal 4 feet thick is workable to a depth deter- 

 mined by the rule above given as follows: 72— 14:48— 14:: 4,000— 

 500:37/33=2,050, which added to 500, the depth limit for a bed of the 

 minimum thickness, gives 2,550 as the depth limit for the bed under 

 consideration. 



The accompanying chart (fig. 3) is copied from part of a large- 

 scale diagram used by the Geological Survey in classifying coal land. 

 It is arranged to show the depth limit fixed for a coal of any B. t. u. 

 value of any thickness under 6 feet. 



For convenience the readings for the even feet and for the even 

 thousand B. t. u. are given in the following table : 



Limits of minahle depth of' coal of various thicknesses and various heating 



values in B. t. u. 



In general the limit of depth at which a coal bed may be profitably 

 mined depends entirely upon the thickness of the bed and the quality 

 of the coal. If, however, a thin but workable bed that lies below 

 its ordinary minable depth is overlain by a thicker bed that lies 

 within its own minable depth, the lower thin bed may, perhaps, be 

 profitably worked by extending downward' the shaft sunk to the 

 higher, thicker coal, whereas the thinner bed could not have been 

 profitably mined alone. A single shaft may also give access to all 

 the coals of a group, and, though the cost of raising all the coal will 

 be the same as if the coal were taken from a single bed, the cost of 

 the shaft and the interest on that cost will be borne by the proceeds of 

 a larger product. In some places the whole cost of a shaft may be 

 borne by the coal of a thick bed that underlies several thinner beds 



