THEORY of the EARTH. 241 



coal is not of that fixed and infufible kind, which burns with- 

 out flame or fmoak, but is bituminous or inflammable coal. 



WE have hitherto been refting the argument upon a fmgle 

 point, for the fake of fimplicity or clearnefs, not for want of 

 thofe circumftances which fhall be found to corroborate the 

 theory. The (Irata of foflil coal are found in almoft every in- 

 termediate (late, as well as in thofe of bitumen and charcoal. 

 Of the one kind is that foflil coal which melts or becomes fluid 

 upon receiving heat ; of the other, is that fpecies of coal, found 

 both in Wales and Scotland, which is perfectly infufible in the 

 fire, and burns like coaks, without flame or fmoak. The one 

 fpecies abounds in oily matter, the other has been diftilled by 

 heat, until it has become a caput mortuum, or perfect coal. 



THE more volatile parts of thefe bituminous bodies are 

 found in their feparate flate on fome occafions. There is a ftra- 

 turn of limeftone in Fifefhire near Raith, which, though but 

 (lightly tinged with a black colour, contains bituminous mat- 

 ter, like pitch, in many cavities, which are lined with calcare- 

 ous fpar cryftallized. I have a fpecimen of fuch a cavity, in 

 which the bitumen is in fphericles, or rounded drops, immerfed 

 in the calcareous fpar. 



Now, it is to be obferved, that, if the cavity in the folid 

 limeftone or marble, which is lined with calcareous cryftals con- 

 taining pyrites, had been thus encrufted by means of the fil- 

 tration of water, this water muft have diflolved calcareous fpar, 

 pyrites and bitumen. But thefe natural appearances would not 

 even be explained by this diflblution and fuppofed filtration of 

 thofe fubftances. There is alfo required,^?, a caufe for the 

 feparation of thofe different fubftances from the aqueous men- 

 ftruum in which they had been diflolved : zdly, An explana- 

 tion of the way in which a diflolved bitumen fhould be formed 

 into round hard bodies of the moft folid ftrudure : and, laftlv, 



ft * \f J * 



Some probable means for this complicated operation being per- 



H h formed, 



