OUTLINES OF GEOLOGY. 89 



fracture of anthracite mask the vegetable form, it is often 

 possible to develop it by careful cleav ng. Coal occasion- 

 ally alternates with the carboniferous limestone, which also 

 contains bitumen, and the carboniferous sandstone contains 

 thin seams of coal. 



(6.) The transition limestone is often completely impreg- 

 nated with carbon and bitumen, and the slate of the subme- 

 dial order contains anthracite imbedded, and also in thin 

 seams. 



(7.) The rocks of the inferior order contain plombago, 

 and although all signs of vegetable structure have disap- 

 peared, analogy would lead us to ascribe that of the inferior 

 slate, at least, to the same origin. 



The changes which are observed in the fossil wood of the 

 modern formations, and which differ only in degree in the 

 lignites and brown coal of the superior and supermedial 

 orders, show that it is only necessary for vegetable matter 

 to be steeped for a sufficient length of time in water, to 

 cause it to assume a bituminous character. In order to ac- 

 count for the formation of true coal and anthracite, it is ne- 

 cessary to call in the action of heat. This, if acting upon 

 brown coal, subjected to pressure, will fuse it, and on cooling 

 there is every probability that it would assume the form of 

 bituminous coal. If the pressure be less intense, the volatile 

 matter would escape, and anthracite would be the result. 

 That the coal measures have been subjected to heat appears 

 from the character of the rocks, whose argillaceous cement 

 is indurated like baked clay, and the source of this heat is 

 apparent in many coal formations, in the presence of trapp 

 and porphyritic rocks, in dykes, and intercalated masses. 



196. The universal prevalence of the formation we have 

 called diluvium over the whole surface of the continents, 

 shows that the present land, after having been raised from 

 beneath the ocean, had been subjected to at least one return 



