CLASSIFICATION Off MiNEHaL LANDS. 105 



ATTITUDB AND SBFTS OP TESI OOAIi. 



As already stated, the coal throughout wide areas lies entirely 

 below the level of drainage — in places hundreds or even thousands 

 of feet below^ — and the outcrops of the particular coal beds on which 

 land is classified as coal land may be scores of miles away. It there- 

 fore becomes necessary for the field man to determine as accurately 

 as possible not only the position of the coal outcrop with reference 

 to the land lines and the thickness of the coal as exposed along the 

 outcrop or the extent of the lenses, but also the depth of the bed 

 beneath the surface and its attitude as it dips into the basin. He 

 does this, first, by studying the inclination of the coal beds where 

 they dip into the ground and, next, by studying the inclination and 

 thickness of the other rocks that overlie them. As coal beds lie 

 more or less nearly parallel with layers of sandstone, shale, and 

 limestone, one of his duties is to determine how nearly the coal beds 

 are parallel with these other rock layers. If he finds by observation 

 at many points that there is very little variation in the interval, say 

 300 feet, between a limestone bed above and the coal bed that he is 

 studying, he may assume that the dip of that limestone bed measured 

 possibly at a point a mile back from the outcrop of the coal indi- 

 cates the dip of the coal beneath that point at a level 300 feet below 

 the surface. If, again, he finds that some other rock bed a thousand 

 feet above the coal bed is parallel to it, he may assume that a measure- 

 ment of the dip of that bed taken 5 or 10 miles back from the coal 

 outcrop may indicate rather closely the dip of the coal bed itself 

 1,000 feet below. Hence the field geologist must not only study the 

 details of the coal bed along its outcrop, but must also study the 

 geology of the area back of that outcrop, especially with reference 

 to the dip of the rocks. If, as sometimes happens, he finds that 

 the space between a coal bed and the overlying rocks is variable, he 

 can not compute the depth of the coal bed so closely. In some places all 

 the rocks associated with certain coal beds, having the same great folds 

 and basins, have been overspread by a blanket of other rocks that 

 do not have the same structure. Such a blanket may completely hide 

 not only the coal-bearing formation but the other formations that are 

 associated with it and that have been folded in the same way. In 

 such places it is only possible to infer from broad general knowledge 

 of the field how deep the coals may lie below the surface. 



dVALm^ OF THE COAIi. 



The character and quality of a coal can be determined in part by 

 a simple examination of the coal bed. Thus it is possible in most 

 examinations to determine whether a coal is a lignite, a subbitumi- 



