394 CONVEESION OF COAL INTO LIGNITE. [Ch- XXV. 



dation, must have measured 900 miles in length, and in some places 

 more than 200 miles in breadth. By again referring to the section (fig. 

 505, p. 390), it will be seen that the strata of coal are horizontal to the 

 westward of the mountains in the region r> e, and become more and 

 more inclined and folded as we proceed eastward. Now it is invariably 

 found, as Professor H. D. Rogers has shown by chemical analysis, that 

 the coal is most bituminous towards its western limit, where it remains 

 level and unbroken, and that it becomes progressively debituminized as 

 we travel southeastward towards the more bent and distorted rocks. 

 Thus, on the Ohio, the proportion of hydrogen, oxygen, and other vola- 

 tile matters, ranges from forty to fifty per cent. Eastward of this line, 

 on the Monongahela, it still approaches forty per cent., where the strata 

 begin to experience some gentle flexures. On entering the Alleghany 

 Mountains, where the distinct anticlinal axes begin to show themselves, 

 but before the dislocations are considerable, the volatile matter is gene- 

 rally in the proportion of eighteen or twenty per cent. At length, when 

 we arrive at some insulated coal-fields (5', fig. 505) associated with the 

 boldest flexures of the Appalachian chain, where the strata have been 

 actually turned over, as near Pottsville, we find the coal to contain only 

 from six to twelve per cent, of bitumen, thus becoming a genuine an- 

 thracite.* 



It appears from the researches of Liebig and other eminent chemists, 

 that when wood and vegetable matter are buried in the earth, exposed 

 to moisture, and partially or entirely excluded from the air, they decom- 

 pose slowly and evolve carbonic acid gas, thus parting with a portion of 

 their original oxygen. By this means, they become gradually converted 

 into lignite or wood-coal, which contains a larger proportion of hydrogen 

 than wood does. A continuance of decomposition changes this lignite 

 into common or bituminous coal, chiefly by the discharge of carburetted 

 hydrogen, or the gas by which we illuminate our streets and houses. 

 According to Bischoff, the inflammable gases which are always escaping 

 from mineral coal, and are so often the cause of fatal accidents in mines, 

 always contain carbonic acid, carburetted hydrogen, nitrogen, and olifiant 

 gas. The disengagement of all these gradually transforms ordinary or 

 bituminous coal into anthracite, to which the various names of splint- 

 coal, glance-coal, hard coal, culm, and many others, have been given. 



We have seen that, in the Appalachian coal-field, there is an intimate 

 connection between the extent to which the coal has parted with its gas- 

 eous contents, and the amount of disturbance which the strata have 

 undergone. The coincidence of these phenomena may be attributed 

 partly to the greater facility afforded for the escape of volatile matter, 

 where the fracturing of the rocks had produced an infinite number of 

 cracks and crevices, and also to the heat of the gases and water pene- 

 trating these cracks, when the great movements took place, which have 

 rent and folded the Appalachian strata. It is well known that, at the 



* Trans, of Assoc, of Amer. Geol. p. 410. 



