104 CLASSIFICATION OF THE PUBLIC LANDS. 



in thickness and in character of the intervening rocks and of the 

 spaces between the beds. In regions in which the beds are not well 

 exposed it may be possible, from a knowledge of the position of a 

 bed with reference to other rocks which show on the surface, to 

 determine exactly the position of the particular coal under considera- 

 tion and, by means of a very small amount of digging, to expose the 

 bed so that its thickness and partings can be measured. In some 

 coal fields hundreds of such openings have been made in the study 

 of the bed. In other fields the beds occur in rocks that weather down 

 to soils rapidly, and it may be only where a coal bed crosses a stream 

 or is otherwise exposed that it can be located or seen. It may be 

 difficult to trace such a bed from point to point, and in some places 

 where exposures are several miles apart it may be difficult or impos- 

 sible to determine exactly its position. In other places beds may be 

 traced readily, but the fact that they have been burned continuously 

 along their outcrops makes it difficult to get accurate information 

 concerning their character and thickness. It is of course possible to 

 map the position of the coal bed in such places and it is then neces- 

 sary, from such information as can be obtained concerning the thick- 

 ness and character of the coal, to infer its character at points between 

 these places. It is especially in such areas that a general knowledge 

 of the " habits " of the bed or group of beds assists in their classifica- 

 tion, for if it is known that a particular bed, whose burned outcrop 

 has been traced with detailed measurements perhaps at only one or 

 two places in a township, occurs at a certain horizon in a certain 

 formation, it is possible to surmise whether it is regular or irregu- 

 lar in the broad area between these exposures, where no information 

 on the bed itself can be obtained. Again, it may be possible, by mak- 

 ing a careful study of a coal in a mining region, to apply the infor- 

 mation to a wide area where but scanty information on the coal 

 itself can be obtained. Wherever the coal is exposed the field man 

 makes careful examination and measurement of every possible sec- 

 tion, measuring down to the fraction of an inch, even though, where 

 the bed is irregular, it is recognized that another measurement made 

 a short distance away may be quite different. Where a bed is irregu- 

 lar special effort is made to obtain as many measurements as 

 possible; in order to obtain average figures for use in the classification 

 and valuation of the land underlain by that bed. In some places 

 Avhere the data are very meager, as in regions where the rocks crum- 

 ble to soil and the land is largely meadow land, consideirable time 

 has been given to making openings on the coal because of the neces- 

 sity of having actual information and measurements as a basis for 

 classification. 



