CONVERSION OF COAL INTO ANTHRACITE. 
403 
fact, I may mention that whenever any ^3art of a swamp in 
Louisiana "is dried up, during an unusually hot season, and 
the wood set on tire, pits are burnt into the ground many 
feet deep, or as far down as the tire can descend without 
meeting with water, and it is then found that scarcely any 
residuum or earthy matter is left. At the bottom of all 
these cypress swamps ” a bed of clay is found, with roots 
of the tall cypress (Taxodium just as the under¬ 
clays of the coal are tilled with Stigmaria, 
Conversion of Coal into 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 decompose slowly and evolve carbonic acid gas, thus 
parting with a portion of their original ox-ygen. 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 carbureted hydrogen, or the gas by which we 
illuminate our streets and houses. According to Bischoff, 
the inflammable gases which are always escaping from min¬ 
eral coal, and are so often the cause of fatal accidents in 
mines, always contain carbonic acid, carbureted hydrogen, 
jiitrogen, and olefiant gas. The disengagement of all these 
gradually transforms ordinary or bituminous coal into an¬ 
thracite, to which the various names of glance - coal, coke, 
hard-coal, culm, and many others, have been given. 
There is an intimate ^connection between the extent , to 
wdiich the coal has in different regions parted with its gas¬ 
eous contents, and the amount of disturbance which the 
strata have undergone. The coincidence of these phenom¬ 
ena may be attributed partly to the greater facility afforded 
for the escape of volatile matter, when the fracturing of the 
rocks has produced an infinite number of cracks and crev¬ 
ices. The gases and water which are made to penetrate 
these cracks are probably rendered the more effective as 
metamorphic agents by increased temperature derived from 
the interior. It^s 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 disengagement of volatile matter from the carboniferous 
rocks. 
In Pennsylvania the strata of coal are horizontal to the 
westward of the Alleghany Mountains, where the late Pro¬ 
fessor H. D. Rogers pointed out that they were most bitu- 
