30 ILLUSTRATIONS OF THE 



fuppofe, that this had adually taken place in 



the bowels of the earth ; and was a fad: which 

 though we were not able to explain it, we were 

 iiot entitled to deny. The principle jult men- 

 tioned relieves us therefore from a difficulty, 

 that would have embarraifed, but could not have 

 overturned, this theory of the earth. 



26. From the arguments which the argilla- 



V 



ceous (Irata afford for the igneous confolidation 

 of foffils, I fliall feled one on which Dr Hutton 

 ufed to lay conliderable llrefs, and which fome 

 of the adverfaries of his fyflem have endeavour- 

 ed to refute. This argument is founded on the 

 ftrudure of certain iron-ftones called feptaria, 

 often met with among the argillaceous fchiftus, 

 particularly in the vicinity of coal. Thefe ftones 

 are ufually of a lenticular or fpheroidal form, 

 and are divided in their interior into diftind 

 fepta, by veins of calcareous fpar, of which one 

 fet are circular and concentric, the other redi- 

 lineal ; diverging from the centre of the for- 

 mer, and diminilhing in lize as they recede from 

 it. Now, what is chiefly to be remarked is, that 

 thefe veins terminate before they reach thefurface 

 of the ftone ; £0 that the matter with which they 

 are filled cannot have been introduced from with- 

 out by infiltration, or in any other way what- 

 foever. The only other fuppofition, therefore, 

 that is left for explaining the fingular ftrudure 



of 



