538 ISJ.E OF MAN. GEOLOGY. SCHIST. 



Among the granite columns lately brought from Leptis, 

 and now in the British museum, it may be remarked that 

 many are decomposing in crusts concentric to the axis of 

 the shaft; each crust varying from one-sixth to one-fourth 

 of an inch in thickness, and still possessing considerable 

 tenacity, with an appearance scarcely changed from that 

 of the fresher specimens. However difficult it may be 

 to explain this mode of decomposition, it appears quite 

 similar to that which occurs, in many cases, in natural 

 blocks of granite, as well as in the trap rocks ; and may 

 perhaps, in some measure invalidate, not only the pre- 

 ceding opinions, but, to a certain degree also, the observa- 

 tions formerly made respecting the schistose granite of 

 Arran, where a reference was made to this fact. 



The next rock in order to the granite is the argillaceous 

 schist, the general extent and disposition of which have 

 already been stated. In describing the leading features 

 and relations of this mass, the general term of schist has 

 been adopted ; since the whole, though differing in many 

 places in mineral composition, bears a common relation 

 to the rocks which follow it, and doubtless also to that 

 which precedes. With such a term, the history of this 

 body of strata will be more intelligible, and the general 

 bearings of the whole more easily apprehended, than if, in 

 describing it, the several varieties were considered sepa- 

 rately ; a mode calculated not only to produce a tedious 

 repetition, but in danger of conveying fallacious ideas 

 respecting its character. As on former occasions, it will 

 be more consistent with clearness of description to consider 

 these several varieties as so many mineralogical distinc- 

 tions ; taking care to notice their relations to the general 

 mass wherever they are such as to present any useful 

 information, or where they tend to illustrate the history 

 of the argillaceous schists. 



This proceeding appears the more necessary, since, as 

 was already hinted in the account of Bute, some obscurity 

 appears occasionally to have been produced by confound- 



