> 4 Fe: We Hats 

306 -- «-DYNAMICAL GEOLOGY.  [Boox IIL 
goes to increase the soil. But sheltered from the atmosphere, — 
-_ exposed to the action of water, especially with an increase of tempe- 
rature, and under some pressure, it is converted into lignite and coal. 
An example of this alteration was observed a few years ago in the 
Dorothea mine, Clausthal. Some of the timber in a long-disused 
level, filled with slate rubbish, and saturated with the mine-water 
from decomposing pyrites, was found to have a leathery consistence 
when wet, but, on exposure to the air, hardened to a firm and 
ordinary browa-coal, which had the typical brown colour and 
external fibrous structure, with the internal fracture, of a black glossy 
pitch-coal.! This change must have been produced within less than 
four centuries—the time since the levels were opened. According to 
Bischof’s determinations the conversion of wood into coal may take 
place, Ist, by the separation of carbonic acid and carburetted 
hydrogen; 2nd, by the separation of carbonic acid and the 
formation of water either from oxidation of hydrogen by meteoric 
oxygen or from the hydrogen and oxygen of the wood; 3rd, by the 
separation of carbonic acid, carburetted hydrogen and water.? The 
circumstances under which the vegetable matter now forming coal 
has been accumulated were favourable for this slow transmutation. 
The carbon-dioxide (choke-damp) of old coal-mines and the carburetted 
hydrogen (fire-damp CH,), given off in such large quantities by coal 
seams, are products of the alteration which would appear to be 
accelerated by terrestrial movements such. as those that compress 
and plicate rocks. During the process these gases escape, and the 
ptoportion of carbon progressively increases in the residue, till it 
reaches the most highly mineralized anthracite (p. 172), or may 
even pass into nearly pure carbon or graphite. In the coal-basins of 
Mons and Valenciennes the same seams which are in the state of 
bituminous coal (gras) at the surface gradually lose their volatile 
constituents as they are traced downward till they pass into anthracite. 
In the Pennsylvanian coal-field the coals become more anthracitic 
as they are followed into the eastern region, where the rocks have 
undergone great plication, and where, possibly during the sub- 
terranean movements, they were exposed to an elevation of 
temperature. Daubrée has produced from wood, exposed to the 
action of superheated water, drop-like globules of anthracite which 
had evidently been melted in the transformation, and which presented 
a close resemblance to the anthracite of some mineral veins.* 
Production of the Schistose Structure.—All rocks are not 
equally permeable by water, nor is the same rock equally permeable 
in all directions. Among the stratified rocks especially, which form. 
so large a proportion of the visible terrestrial crust, there are great 
differences in the facility with which water can travel, the planes of 
sedimentation being naturally those along which water passes most — 
* Hirschwald, 7. Deutsch, Geol. Ges. xxy. p. 364. 
* Bischof, Chem. Geol. i. p. 274. 
* Daubrée, “ Géologie Expérimentale,” p, 463. ‘ Op. cit. p. 177. 
