CLASSIFICATION OF MINEEAL LANDS. 



79 



culty due to the thin and weak roof is also encountered in mining on 

 the outcrop. If only a thin edge or corner of coal extends into a forty 

 it may be neglected, as it will give the tract no value as coal land. If 

 the coal outcrops in bluffs or steep hillsides or has a high dip weath- 

 ering may be neglected. If the coal outcrops at the top of a mesa or 

 on the face of a long, gentle slope an allowance is made for thinness 

 of cover. The possibility of mining outcrop coal by stripping with 

 steam shovel or otherwise is generally considered, as such a method of 

 coal mining has in many places proved highly successful. 



In many areas of the West the coal beds have been burned along 

 the outcrop. In some places one or more or all of the beds have been 

 burned not only along an outcrop over a whole field but under the 

 flat tops of mesas where the coal is close to the surface. In a few 

 places mining has shown that this burning extends back 1,000 feet 

 or more and lies many hundred feet under covert In general, how- 

 ever, the coal does not appear to have burned back more than a 

 few rods. All places where coal has been burned are indicated on 

 the field maps. In classifying the land in these places the edge of 

 the coal is fixed along a line drawn back of the outcrop, only slightly 

 back of it where the line crosses ravines, but to a considerable dis- 

 tance back of it where it crosses projecting divides, especially if the 

 coal is under light cover. 



METHODS or VALUATION.^ 

 COST OF COAIi ZN THE GEOTJITD. 



The total cost of coal in the ground consists of the original pur- 

 chase price, interest on investment, amortization charges, taxes, and, 

 if the coal is leased, the cost of inspection and of collecting royalties. 

 The following table shows the amortization and ultimate cost of coal 

 in the ground per dollar of investment for periods ranging from 20 

 to 40 years, at 5 and 6 per cent interest. 



Ultimate cost of coal per dollar of purchase price. 



1 For a more detailed account of the principles of valuation of public coal land see 

 Survey BuUetln 424, on the valuation of public coal lands. 



