94 CLASSIFICATION OF THE PUBLIC LANDS. 



BFKEIOI OF IHrXBUBIONS. 



Intrusions of igneous rock may affect a coal bed favorably, as 

 where they occur in sheets close above or below the coal and have 

 had the effect of changing it from a bituminous or lower grade of 

 coal to an anthracite. The physical diaracter and B. t. u. value 

 of such coals reflect amply the favorable results of the intrusion. 

 More often the intruded masses of igneous rock are in the form of 

 more or less vertical dikes that cut the bed in various directions. 

 Obviously these dikes detract from the value of the bed by increasing 

 the cost of mining, and an allowance is made in the final price in 

 accordance with their character and extent. 



AU^OWAIiTCE FOB OTHEB FACTOBS. 



In addition to the factors that have been specifically considered, 

 many other factors are taken into account in valuing the land in 

 any field. Of these the one for which the largest allowance is likely 

 to be made is that of uncertainty. In some fields it has been possible 

 to obtain measurements on the coal along its outcrop at points a 

 quarter of a mile or less apart, and some of these measurements 

 are supplemented by others made in drillings at points " back " from 

 the outcrop, so that in such places it is possible to determine the 

 " habit " of the bed with some degree of certainty. In other places, 

 especially in coal fields covered with a blanket of glacial deposits, 

 or in broad valleys where the coals are under an alluvial cover, or 

 in other fields where, owing to almost continuous burning along 

 the outcrops, it is difficult to get accurate information as to the thick- 

 ness of the beds, an allowance is made for uncertainty — an allow- 

 ance that may take the form of a greatly reduced estimated thiclmess 

 of the bed or, as is more common, of a restriction of the assumed 

 area of workable coal. It has been found that the coal of certain 

 fields was originally deposited with more regularity in some belts 

 and areas than in others, and in many places the coal was deposited 

 with great regularity around the edge of a basin but not in its 

 center. Allowance is made for the possible thinning of the coal in all 

 such places if exact data are not at hand, and the tract is classed 

 either as noncoal land or as coal land of the minimum value. Many 

 other factors are taken into account, such as known poor roof or 

 floor, which may seriously affect the cost of mining and the value 

 of the coal in place. Allowance is made for the extra cost of mining 

 beds that include partings by making deductions for partings, it 

 being assumed that the extra cost due to the partings eats up the 

 profits on an equal thickness of coal. 



