CLASSIFICATION OF MINERAL LANDS. 109 



is studied as a whole to determine from point to point the number 

 of beds and their aggregate thickness and value. 



4. The basing value per ton of the coal is computed by comparing 

 its analyses and its obvious character with the standard scale. An 

 average B. t. u. value having been, fixed for the coals of the field 

 or for the different groups of coals, the other qualities of the coal — 

 such as its adaptability to coking or to stocking — are studied to de- 

 termine whether they enhance that value or detract from it, and the 

 price is raised or lowered accordingly. 



5. If the field is small and the data are scattered the field may be 

 studied as a unit. If the data are abundant in any township the 

 coals in that township are studied by themselves, and, exceptionally, 

 the variation in thickness may make it necessary to divide a town- 

 ship into belts or areas, in each of which a basing value of the 

 groups is determined. 



6. All sections of the coal are examined, and if the coal beds are 

 split the equivalent thickness of a solid bed is computed and a memo- 

 randum of the results is placed beside the drawn section. Then 

 further reduction is made for the reduced value of the bed, if it is 

 less than 6 or more than 10 feet thick. If a bed is variable in thick- 

 ness but varies so regularly that lines of equal thickness can be drawn 

 on a map of the field these lines are so redrawn as to indicate the 

 reduced thickness determined by allowance for their reduced value 

 if the thickness is less than 6 or more than 10 feet. If the beds are 

 of irregular thickness the average of the measurements taken is ob- 

 tained, and by the use of the " modulus of irregularity " a computed 

 average thickness is obtained for use in valuation. 



7. The outcrops of the coals are examined to determine which are 

 workable coals and especially to make note of the outcrop of the 

 lowest bed or the bed covering the largest area. Where the coal lies 

 in one or more lenses a computation of its gradation in thickness is 

 made between points at one of which the coal is below the minimum 

 thickness and at the other is above, to determine the position of the 

 minimum. The limits of workable coal having been determined for 

 certain points, the limits of the lenses of the coal are drawn on the 

 maps. For isolated measurements at points where the bed is above 

 the minimum limit the extent and character of the several lenses 

 measured are determined, special formulas being applied where 



possible. 



8. If the coal passes below the depth limit of workable coal the 

 position of that limit is determined and indicated on the map. 



9. The limit of workable coal having now been determined, a line 

 is drawn on a plat along the 40-acre lines, or, where necessary, along 

 the 10, 5, or 2i acre lines in the forty, to separate the area that con- 



