384 



ILLUSTRATIONS OF THE 



* / 



Hone get by decompofition a granulated texture 

 at the furface, fo as hardly to be diftinguifhed 

 from real fandftone. 



Some of the kinds of primary fchiftus alfo, par- 

 ticularly the argillaceous, when much indurated 

 have in their flruclure a confiderable refemblance 

 to whinftone ; they are llightly granular, or la- 

 minated, and have a tendency to a fuarrv texture 



Where it 



appens that this fort of fchiftus and 



whinftone are contiguous, it is natural to exped, 

 that their common boundary will be traced with 



difficulty, and 



many parts will be quite 



certain. Still, however, if a careful examination 

 is made ; if the effeds of accidental caufes are 

 removed ; and, above all, if the more ambiguous 



ft; 

 id 



are 



pared with th 



deciiive 



preted by 



though 



■T, 



mens may be doubtful, we will hardly ever find 

 that any uncertainty remains with refped to en- 

 tire rocks. 



251. This general fad, whidi I ftate on much 

 better authority than that of my own obferva- 

 tions, viz. on thofe of Dr Hutton, is not given 

 as abfolutely without exception. The theory 

 of whinftone which has been laid down here, 

 leads us indeed to look for fome fuch exceptions. 

 It is certain, that the bafts of whinftone, or the 



material out of which it is prepared by the ac- 

 tion 



