Ch. XXV.] CONVERSION OF LIGNITE INTO COAL. 503 



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 become progressively 

 debitumiuized as we travel southeastward towards the more bent and 

 distorted rocks. Thus, on the Ohio, the proportion of hydrogen, 

 oxygen, and other volatile matters, ranges from 40 to 50 per cent. 

 Eastward of this line, on the Monongahela, it still approaches 40 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 generally in the proportion of eighteen or twenty 

 per cent At length, when we arrive at some insulated coal-fields (5', 

 fig. 552) 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 6 to 12 per cent, of bitumen, thus 

 becoming a genuine anthracite.* 



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 de- 

 compose slowly and evolve carbonic acid gas, thus parting with a por- 

 tion of their original oxygen. By this means, they become gradually 

 converted into lignite or wood-coal, which contains a larger propor- 

 tion 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, car- 

 buretted hydrogen, nitrogen, and olifiant gas. ■ The disengagement of 

 all these gradually transforms ordinary or bituminous coal into anthra- 

 cite, 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 inti- 

 mate connection between the extent to which the coal has parted with 

 its gaseous contents, and the amount of disturbance which 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 

 present period, thermal waters and hot vapors burst out from the earth 

 during earthquakes, and these would not fail to promote the disen- 

 gagement of volatile matter from the carboniferous rocks. 



Continuity of seams of coaL — As single seams of coal are continuous 



* Trans, of Assoc, of Amer. Geol., p. 470. 



