302 THE BORDERLAND OF SCIENCE. 



as well as of the formation of peat, which must be 

 regarded as only needing submersion and consequent 

 pressure to become, in the lapse of time, either true 

 coal or very near akin to it.* Yet, it remains true 

 that the carboniferous group is the coal group par 

 excellence ; and when to this consideration is added 

 the enormous thickness of the series of strata included 

 in this group, we seem justified in concluding that this 

 long period was characterised by some remarkable and 

 distinctive peculiarity. 



Now, whether we consider the lower portion of the 

 carboniferous series, remarkable for the masses of lime- 

 stone derived chiefly from animal substances, or the 

 upper, where the coal-seams or vegetable layers 

 abound, we find evidences of the presence of enormous 

 quantities of carbon. In the upper part, the mere 

 existence of a most abundant vegetation implies the 

 presence of vast quantities of carbonic acid gas in the 

 air. It seems not unlikely that this gas escaped from 

 subterranean regions through the outlets formed by 

 volcanoes ; and the idea is suggested that the carboni- 

 ferous period was one of great volcanic energy. In 

 the older periods, there was probably a greater degree 

 of subterranean activity, and from the carboniferous 



* ' Near the surface,' says Professor Ansted, ' this substance (peat) 

 is light-coloured and spongy, and the vegetable matter is little altered ; 

 deeper, it is brown, dense, and decomposed ; at the bottom, it is black 

 and nearly as dense as coal.' As a fuel, however, peat contains much 

 ash. The same is true of lignite and brown coal. Moreover, brown 

 coal is injured by exposure to the weather, which is not the case with 

 true coal. Lignite splits in the air ; brown coal falls to powder. 



