CLASSIFICATION OF MINEEAL LANDS. 93 



If the dip of the coal bed whose value is given above had not 

 been regular the rate of its diminishing value would have been 



^TT =$1 for each 33^ feet, so that if the depth under a given 



forty averaged 1,600 feet, the value of the forty would be 



^60-^-^^^=127. 



If more than one bed of coal underlies a 40-acre tract and the beds 

 are not close together each one is valued independently and the value 

 of the forty is taken as the sum of the value of the independent beds. 

 If several beds occur in a group a normal value is figured out for 

 the group and this value is reduced according to the depth. In fixing 

 the value of a group of dipping beds allowance is made for the 

 change in the value of the group due to the lower beds passing below 

 the 500-foot line before the higher beds and reaching the depth limit 

 before they do. 



Formulas have been worked out to facilitate the calculation of the 

 value of beds involved in some of the more common types of 

 computations in these cases are read directly from large diagrams. 

 These diagrams contain so much matter that a reproduction of one 

 of them (fig. 6) on the reduced scale necessary for publication in 

 this bulletin does little more than give a general idea of their char- 

 acter, particularly as they are too involved to permit their descrip- 

 tion in a bulletin that is not intended to set forth details of classifica- 

 tion. 



AIiIiOWAIirOB FOB FAUIiTS. 



Faults — that is, breaks in the rocks involving a movement of beds 

 on either side of the break — occurring in a coal field depreciate the 

 value of the coal for mining and in places may render it entirely 

 unworkable. Small faults may be disregarded unless they are so 

 numerous as to render the coal expensive or difficult to mine. If, 

 however, the throw of the fault is large — that is, if the edges of 

 the beds that are broken have been so far separated by movement 

 at an angle to the bedding that entries can not handily be driven from 

 the coal on one side of the fault to the coal on the other — ^the value 

 of the land is clearly less than that in which there is no fault, its 

 smaller value being due both to the extra cost of recovery and to 

 possible crushing of the coal along the fault planes. The price of 

 coal in such places is determined by considering the two parts of 

 any forty traversed by the fault separately, and giving the normal 

 price to the part having the higher value and 40 to 95 per cent of 

 the normal price to the part having the lower value. From the total 

 thus obtained 5 to 15 per cent may be deducted to allow for crushing 

 along the fault plane. 



