72 . CLASSIFICATION OF THE PUBLIC LANDS. 



The criteria given in the preceding paragraphs are intended to 

 determine the workability or nonworkability of a coal measured at 

 any given point. The most difficult problem in classification is the 

 determination of the probable limits of workability of an irregular 

 coal. Coal occurs in beds ranging in thickness from a fraction of 

 an inch to 100 feet or more and in areal extent from a few square feet 

 to thousands of square miles. Some beds maintain a nearly uniform 

 thickness over hundreds of square miles. Others may be traced at 

 the same geologic horizon over hundreds or thousands of square 

 miles but vary greatly in thickness, ranging from a few inches to 

 several feet or back again within a quarter of a mile. Every grade of 

 regularity or irregularity between these extremes is found, and it is 

 possible to determine the " habit " of some beds as regards regularity. 



The simplest problem is that in which two unequal measurements 

 on a single bed, taken at different places, are available. All such 

 beds are assumed to grade uniformly in thickness from the thicker 

 measurement to or through the thinner measurement, and a limit 

 to the workable coal is thus fixed that, while it may or may not agree 

 with the unknown facts, is the most probable limit. In general, a coal 

 bed that can be traced continuously along the face of a cliff, as can 

 many beds in the West, has usually been assumed to extend under 

 the land back from the cliff at least one-half the length of the cliff 

 outcrop, the lens or bed having the shape of a half -circle, the length 

 of the cliff outcrop being the diameter of the circle. Obviously, if 

 the outcrop runs along the cliff for many miles, the extension of the 

 coal back from the outcrop may be modified by many other factors, 

 such as limit of depth, or outcrop on the opposite side of the hill. 

 If the bed outcrops along the cliff with irregular thickness, only a 

 moderate extension of the bed beiiind the cliff is assumed, the esti- 

 mate being based on the character of the irregularities shown by 

 measurements made along the cliff, or by a general knowledge of the 

 extent of the lenses of that particular coal or of coals of that group, 

 and depending on the general " habit " of the bed, if known, and 

 also taking into account all local features. If the exposed outcrop 

 does not extend in a straight line, but, as is more common, runs in 

 and out of ravines, careful note is made of the thickening or thin- 

 ning of the coal between one point of measurement and another in 

 order to detect, if possible, any general tendency of the bed to thicken 

 or thin in any direction, and all these measurements and tendencies 

 are taken into account in determining the probable extent of the 

 lens in any direction. 



Many beds studied are known to be of less than workable thick- 

 ness in larger or smaller part, and any measurement showing a work- 

 able thickness on such a bed must usually be considered as a meas- 

 ure taken at the center of a small lens of workable coal. A discovery 



