102 CLASSIFICATION OP THE PUBLIC LANDS. 



tances, or it may consist of the presence just above or below the bed of 

 & stratum that has easily recognizable characteristics. In order to 

 trace and identify some coal beds it is necessary to study their rela- 

 tions to one another. One of the main reasons for studying the 

 stratigraphy is to identify if possible at the outset the group of coals 

 studied, and in this work note should be taken of the determinations 

 reached by the wide study of the same formations in previous seasons. 

 For example, in the great coal field that covers much of western North 

 Dakota and part of South Dakota, all of southeastern Montana and 

 the great Powder River field of Wyoming, there are two coal- 

 bearing formations — ^the Fort Union above, named from its early 

 recognition at Fort Union in North Dakota, and the Lance forma- 

 tion below, named from the occurrence of those rocks on Lance 

 Creek in Wyoming. The Fort Union coals are as a rule persistent; 

 they have local thicknesses of 10 to 30 feet or more, and some of 

 •them can be traced for hundreds of miles along their outcrop, and 

 individual beds can be recognized from point to point. On the other 

 hand, the Lance coals occur as a rule in small lenses, most of them 

 a fraction of a mile or a very few miles in extent. The coal in these 

 lenses ordinarily is but little above the minimum workable limit and 

 thins rapidly to nonworkable thickness in all directions. If the 

 geologist knows that the coals in any particular area are in the Fort 

 Union or Lance formation he knows what to expect concerning them, 

 for if he finds that they are of Lance age and he is studying at the 

 moment the coal at a point where it is workable, he is careful to 

 trace it if possible in order to see how far it may extend before it 

 becomes unworkable, so as to determine as closely as practicable the 

 extent of that particular lens. If, on the other hand, he knows 

 that the coal before him is of Fort Union age he attempts to de- 

 termine, if possible, what particular coal bed in the Fort Union it 

 is, and, assuming that he will find that same bed in a large part of the 

 country ahead of him and that he will judge of its thiclmess by com- 

 bining a great number of measurements made over a large territory, 

 he does not with the same attention attempt to discover whether the 

 coal pinches out a short distance on either side of the point where he is 

 standing. In all of this work it is, of course, not safe to assume too 

 much, for toward the south end of the Powder River field the beds of 

 coal in the Lance formation increase in extent and thickness^ so that 

 they more nearly resemble those of the Fort Union formation, farther 

 north. 



It is not advisable to assume absolutely that, because in some area 

 already examined a coal bed is very extensive and keeps the same 

 thickness with great regularity, it will continue to be extensive and 

 regular in territory that may be studied later. For example, the 

 Pittsburgh bed, which maintains a very uniform thickness over a 



