224 Dr. Mac Culloch on the Lignites. 



Because these rocks are often connected with conchiferous strata, 

 and because they have been supposed never to contain air- vesicles, 

 like the lavas, it is conceived that they are of a submarine nature, 

 or, at least, that they have been formed under the rocky strata, 

 or under some other considerable pressure. It is not very easy 

 to understand how, under some of these circumstances, they could 

 have entangled wood, though fragments of this might have been 

 present at the bottom of the sea ; either from ordinary transporta- 

 tion on the surface, or enclosed in the loose alluvial matters, and 

 possibly in the previous state of lignite. I have already shewn, in 

 other places, that the trap rocks have, like lavas, sometimes 

 flowed on the surface of the land, since they contain vesicles, and 

 that they are then the remaining rocks of ancient and far distant 

 volcanoes, of which all the looser and more destructible testi- 

 monials have been removed by the lapse of time. Under these 

 circumstances, it is not difficult to understand how they may have 

 entangled lignites, or wood imbedded in alluvial strata ; and thus 

 perhaps it is most easy to account for these cases, and particularly 

 for those in which the lignites are found in tufaceous strata sur- 

 rounded by trap. But it is also plain, that, as in the case from 

 Meissner just quoted, the irruption of trap into strata already 

 containing lignites, of whatever age, might have produced similar 

 appearances ; the clays or marls undergoing the changes which 

 commonly result from this cause, while the vegetable or coaly 

 substances would thus become involved in the trap itself, or in 

 the rocks which it had modified. 



It is now necessary to consider the chemical nature of the lig- 

 nites, and to state the circumstances in which they vary from peat 

 on the one hand, and from coal on the other. This examination, 

 I must premise, relates, however, chiefly to those which still par- 

 take of the nature of wood, or which are lignites, mineralogically 

 speaking. Where they are but geologically such, and have become 

 true coal in character, they require no distinct notice. As I have 

 shewn that they approximate to the former substance through the 

 intermediate stage of submerged wood, and that a sort of grada- 

 tion may consequently thus be traced from the living vegetables 



