AGE OF COAL. 217 



smaller than in the Devonian, as it was smaller then than 

 in the Silurian. It grew smaller from the additions 

 which were made in successive periods to the perma- 

 nent land of the continent. There was another point of 

 difference in the sub-carboniferous sea the crinoids 

 were predominant, so that Dana speaks of it as the Cri- 

 noidal sea, while in the Devonian it was. the corals that 

 did most of the work, and in the Silurian, brachiopods, 

 corals, and crinoids were about equally present. 



313. How Coal was Made. All the coal in the world 

 has been formed from trees and other plants. Proof of 

 this has been adduced in 41, and need not be dwelt 

 upon now. The coal is the result of a decomposition, to 

 a greater or less degree, of the woody fibre or substance 

 of the wood. When wood is burned in the open air all 

 the carbon is dissipated by gaseous combinations ; but 

 when the combustion is effected with the air mostly ex- 

 cluded, the carbon is retained to a considerable extent, 

 making charcoal. For a full explanation of this I refer 

 you to Part II. The same changes that occur in the 

 combustion that produces charcoal may occur, under cer- 

 tain circumstances, without the phenomenon of combus- 

 tion. If, for example, the decomposition take place slow- 

 ly under water, the chemical changes are essentially the 

 same as when what is ordinarily called combustion is 

 seen. If you observe what the composition of wood is, 

 you can see how this can be. Wood is composed of 

 carbon, hydrogen, and oxygen, the elements which are 

 engaged in ordinary combustion ; and it is by the slow 

 action of these upon each other that the slow combus- 

 tion under water goes on. Very properly, then, is this 

 process called eremacausis, this term being made from 

 two Greek words, erema, slowly, and kamis, burning. 

 The same thing occurs in the decay of vegetable matter 

 in the open air, the slow combustion in this case being, 

 however, carried out in full, as in the ordinary burning 

 of wood, the carbon, oxygen, and hydrogen all forming 



K 



