412 The American Geologist. June, 1894 
ways. Just which method lias assumed greatest prominence 
in each particular case is difficult of ready solution, since it is 
quite probable that in most instances several very different 
agencies have acted simultaneously. 
The changes in character which vegetal materials undergo 
when shut off from the free access of the air. in the presence 
of moisture, with an even temperature or increased heat, and 
under some pressure, are very different from those which oc- 
cur when the mass is exposed to ordinary atmospheric influ- 
ences. Instead of rapid oxidation, as in the latter case, the 
various chemical components react among themselves, 
producing finally ordinary coal. The researches of Bischof* 
show clearly that the conversion of woody matter into coal is 
accomplished in at least three ways. By each method the 
carbonic acid escapes; the hydrogen goes off either as C II , 
or H 2 0, in the latter case with or without the aid of meteoric 
oxygen. Both mining operations and stratigraphical work upon 
individual coal beds indicate plainly that a loss of the hydro- 
carbons is constantly going on ; that this loss is comparatively 
speedy in the beginning, or for a short period after the vegetal 
mass is buried; but that gradually the rate becomes less and 
less rapid, until in the end the escape of the volatile matter 
ceases almost altogether. Additional pressure arising from 
the deposition of sediments upon the plant beds naturally 
hastens the normal process of eliminating the continually 
generated gases. Increased temperature likewise tends to 
have the same effect. Still more favorable to the production 
of like results is the action of orogenic movements. 
In a recent paperf Stevenson has advanced some views on 
the anthracite of Pennsylvania which differ materially from 
the commonly accepted opinions concerning the genesis of the 
beds. In substance, they are that from the northeastern cor- 
ner of the great coal bed the old marsh extended westward on 
advancing deltas, and that, consequently, along the marginal 
portions of the beds the plant accumulations, being less rap- 
idly covered up, were subjected to the ordinary influences of 
decomposition for a much longer time than in the more central 
portions. In the middle of the swampy region common 
♦Chemical Geology, vol. i, p. 274. 
fBulletin, Geol. Soc. America, vol. v. pp. 39-70; Nov.. 1893. 
