558 DOMAIN VI. CARBONACEOUS. 



or four are most commonly found, and in general 

 of nearly an equal thickness. 



" 5. Each bed of coal is separated from the 

 others by several rocky strata, which are nearly 

 the same in all coal mines. 



" Those which form the roof and the wall, 

 are always of a schistose argillaceous substance, 

 a kind of friable schistus, almost always sulphure- 

 ous : afterwards follow strata of micaceous sand- 

 stone, which seem derived, at least in part, from 

 the detritus of the primitive mountains of the 

 neighbourhood. 



" These strata of sandstone are often separated 

 by small schistose layers, which contain some 

 symptoms of coal ; they are both often repeated 

 between two beds of coal. 



" It is a general observation, and almost with- 

 out exception, that the schistose layers, and 

 especially those which serve as a roof to the 

 coal, bear impressions of vegetables, particularly 

 capillaria, ferns and reeds, for the most part ex- 

 otic. This circumstance has led several natural- 

 ists to think, that coal itself is composed of the 

 remains of vegetables ; but this opinion appears 

 to me to present great difficulties *."f 



* " One of the facts, which is most opposite to it, is the obser- 

 vation made at Santa-Fe-de-Bogota, by the naturalist le Blond, who 



f Patrin, Min, v. 317. 



