108 TOWN GEOLOGY. [IT. 



the theory that coal has been once vegetable fibre, for 

 it shows how vegetable fibre can, by the laws of 

 nature, become coal. And it certainly helps us to 

 believe that a thing has been done, if we are shown 

 that it can be done. 



This fact explains, also, why in mines of wood- 

 coal carbonic acid, i.e. choke-damp, alone is given off. 

 For in the wood-coal a great deal of the hydrogen still 

 remains. In mines of true coal, not only is choke- 

 damp given off, but that more terrible pest of the 

 miners, fire-damp, or explosive carburetted hydrogen 

 and olefiant gases. Now the occurrence of that fire- 

 damp in mines proves that changes are still going on 

 in the coal : that it is getting rid of its hydrogen, and 

 so progressing toward the state of anthracite or culm 

 stone-coal as it is sometimes called. In the Penn- 

 sylvanian coal-fields some of the coal has actually done 

 this, under the disturbing force of earthquakes; for 

 the coal, which is bituminous, like our common coal, 

 to the westward where the strata are horizontal, 

 becomes gradually anthracite as it is tossed and torn 

 by the earthquake faults of the Alleghany and 

 Appalachian mountains. 



And is a further transformation possible ? Yes ; 

 and more than one. If we conceive the anthracite 

 cleared of all but its last atoms of oxygen, hydrogen, 

 and nitrogen, till it has become all but pure carbon, it 

 would become as it has become in certain rocks of 

 immense antiquity, graphite what we miscall black- 

 lead. And, after that, it might go through one 

 transformation more, and that the most startling of all. 

 It would need only perfect purification and crystal- 

 lisation to become a diamond; nothing less. We 



