272 Tlie Origin and Relations of the Carhon Minerals. 



with lignites, we find that at different ge()h)gical levels they ex- 

 hibit different stages of this distillation — the Tertiary lignites 

 being usnally distinguished without difficulty by the presence of 

 a larger quantity of combined water and oxygen, and a less 

 quantity of carbon, than the Cretaceous coals, and these in turn 

 differ in the same respects from the Triassic. 



All the coals of the Tertiary and Mesozoic ages are grouped 

 under one name ; but it is evident that they are as different from 

 each other as the new and spongy from the old and well-rotted 

 jieat in the peat-bog. 



Coal. — By mere convention, we call the peat which accumu- 

 lated in the Carboniferous age by the name of bituminous coal ; 

 and an examination of the Carboniferous strata in different 

 countries has shown that the peat-beds formed in the Cai'boni- 

 ferous age, though varying somewhat, like others, with the kind 

 of vegetation from which they Avcre derived, have a common 

 character by which they may be distinguished from the more 

 modern coals ; containing less watei', less oxygen, and more car- 

 bon, and usually exhibiting the property of coking, which is rare 

 in coals of later date. Though there is great diversity in the 

 Carboniferous coals, and it would be absurd to express their 

 composition by a single formula, it may be said that, over the 

 Avhole world, these coals have characteristics, as a group, by 

 which they can be recognized, the result of the slow decomposi- 

 tion of the tissue of plants which lived in the Carboniferous age, 

 and which have, by a broad and general change ajoproximated to a 

 certain phase in the spontaneous distillation of plant-tissue. 

 An experienced geologist will not fail to i-efer to their proper 

 horizon a group of coals of Carboniferous age, any more than 

 those of the Cretaceous or Tertiary. 



Anthracite. — In the ages anterior to the Carboniferous, the 

 quantity of laud vegetation was apparently not sufficient to form 

 thick and extensive beds of peat; but the remains of plant-tissue 

 are contained, in all the older formations, though there only 

 as anthracite or graphite — the last two groups of residual 

 products. Of these we have examples in the beds of graphite in 

 the Laurentian rocks of Canada, and of anthracite of the Lower 

 Silurian strata of Upper Church and Kilnaleck, Ireland. 



