THE CARBONIFEROUS SYSTEM. 121 



This is the condition in which we find most of the beds of peat and 

 lignite that accumulated in what is called the Carboniferous age, millions 

 of years ago, and which, deeply buried, have been subjected to a slow 

 and general distillation, resulting in the different varieties of bitumin- 

 ous coal. Where exposed to peculiar influences, as to heat from volcanic 

 eruptions, or from the elevation of mountain chains, where all the strata 

 are metamorphosed, the volatile constituents of bituminous coal are par- 

 tially or perfectly driven off, giving us, first, semi-bituminous coal, then 

 anthracite, and finally graphite. The process by which graphite and 

 anthracite are formed from ordinary bituminous coal is indicated in the 

 succeeding formulae : 



Bituminous Coal. Loss. Anthracite. 



Carbon 18.10 3.57 14.53 



Hydrogen 1.20 0.93 0.27 



Oxygen 2.07 1.32 0.65 



Anthracite. Loss. Graphite. 



Carbon 14.53 1.42 13.11 



Hydrogen 0.27 0.14 0.13 



Oxygen 0.65 0.65 0.00 



All the varieties of coal mentioned above shade into each other, and 

 we have lignites that exhibit every degree of approach to bituminous 

 coals ; semi-bituminous coals intermediate between bituminous coal and 

 anthracite and graphitic anthracite, by which the anthracites are con- 

 nected with graphite. 



The geological portion of the different varieties of coal accords with 

 the theory of their origin given above. For example : the oldest rocks 

 known, contain only the residual products of the distillation of vegetable 

 tissue, graphite and anthracite. In the Carboniferous age the terrestrial 

 vegetation was luxuriant over large areas, and conditions prevailed 

 favorable to the formation of beds of peat. * These, submerged and deeply 

 buried under sediments which were deposited upon them, have, as a 



* Judging from the circumstances in which the most extensive deposits of peat are 

 produced at the present time, we may infer that the climate was moist and equable, 

 but neither very hot nor cold, since in tropical climates vegetable tissue runs through 

 all its changes so rapidly that but but little accumulates in a bituminized state, while 

 in a cold climate vegetation is stinted, and there is but little of it to be preserved. It 

 has been suggested that in the Carboniferous age the atmosphere contained much 

 more carbonic acid than now. But of this no proof is given except the succulent and 

 luxuriant vegetation, while the great numbers of air-breathing animals represented 

 by remains found in the Carboniferous rocks indicate that the atmosphere was not 

 greatly different from what it now is. 



