CLASSIFICATION OF MINERAL LANDS. 73 



or measurement of a coal bed by a well, or by drilling, will have the 

 same value as an isolated measurement on the outcrop. The content 

 and shape to be assigned to any lens must depend on conditions and 

 are largely matters of judgment, and such assignments or determi- 

 nations especially require a wide knowledge of coals and of the par- 

 ticular group of coals to which the bed under consideration may 

 belong. To insure unifonnity in treating the thousands of ques- 

 tions of this kind that may arise — for nearly every field involves 

 some, and many fields involve a great number — the Geological Sur- 

 vey has gradually established a series of precedents, and in every 

 question the attempt is made to reach conclusions by means of mathe- 

 matical calculation. The limits of this bulletin will not allow the 

 consideration of these methods here. 



Some of the facts about the deep mining of coal in this country 

 and abroad have been presented and discussed in an earlier bulletin 

 of the Survey.^ Not many years ago it was the common opinion 

 and practice in parts of the West to consider as coal land only land 

 on which coal actually cropped out, and requests are still occasionally 

 sent to the Geological Survey asking the reclassification of certain 

 land that had been previously classified as coal land, the writers 

 contending, and supporting their contention by abundant affidavits, 

 that the land is noncoal, because no coal shows at the surface and 

 none has been found on it in wells. If such land should be consid- 

 ered noncoal land, however, most of the coal now mined in Illinois, 

 Indiana, western Kentucky, Michigan, Kansas, and some other States 

 comes from noncoal land, for most of the mines in those States are 

 on lands where the coal mined does not crop out but is reached by 

 shafts. The actual outcrop of most of the coal mined in these States 

 is from 5 to 75 miles away from the shafts. 



Two questions are here involved : (1) How deep can coal be mined ? 

 (2) Can all coals be mined to the same depth? Coal is now being 

 mined to a depth of practically 4,000 feet (3,937 feet), and many 

 shafts in England, Belgium, France, and Germany go deeper than 

 3,000 feet. Moreover, a consideration of the still greater depths to 

 which substances other than coal are being successfully mined and 

 a study of the deepest coal mines have convinced many of the best 

 engineers of England and Europe that coal mining will be extended 

 to 5,000 feet.^ The fact also that some of the largest coal com- 

 panies of this country have purchased as coal land certain tracts 

 under which the coal is believed to lie at depths of 5,000 to 6,000 feet 



1 Fisher, C. A., Depth and minimum tliicknesis of beds as limiting factors In valuation 

 of coal lands : Bull. U. S. Geol. Survey No. 424, 1910, pp. 48 et seq. 



2 Idem, p. 51. 



