igt 



ILLUSTRATIONS OF THE 



peeled, if that coal was deprived of its bitumi- 



nous part by fubterraneous diftillation *. 



Til is, 



however, admits of explanation. Though a ge- 

 neral connection, on the above hypothecs, might 

 be expeded between bitumens and infulible 

 coal, we cannot look for it in every inftance. 

 The heat which drove off the bitumen from 

 one part of a llratum of coal^ may only have 

 forced it to a colder part of the fame llratum ; 

 and thus, in feparating it from one portion of 

 carbonic matter, may have united it to another. 



Blind-coal may therefore be found where no 

 bitumen has been adlually extricated. In like 

 manner, bitumen may have been feparated, where 

 the coal was not reduced to the flate of coak, as 

 a part of the bitumen only may have been driven 

 off, and enough left to prevent the coal from 

 becoming abfolutely infuhble. 



It fhould be coniidered too, if the bitumen 

 was really feparated, and forced, in the Hate of 

 vapour, into fome argillaceous or limeilone (Ira 

 turn, that this llratum may have been walled and 

 worn away long ago, fo that the bitumen it con- 

 tained may have entirely difappeared. It does 

 not therefore neceffarily follow, that, wherever 

 we find blind' coal, there alfo we iliould difcover 

 forae of the purer bitumens. 



Note 



* Geol. Effajs, p. 473 



