460 BUTE. GEOLOGY. 



mixtures always equally frequent and irregular. That 

 portion of the belt however which reaches from Arran 

 to Loch Lomond, exhibits the whole of them, as they 

 have now been described, in almost every part; and 

 they will also be found to occur in many places to the 

 eastward of this limit. Towards Dunkeld the series is 

 more simple, presenting little else but the argillaceous 

 and gray wacke schists ; although the recurrence of these 

 two members is here as irregular as where the series 

 is more complicated. It may also be remarked, that 

 in this case the boundary towards the micaceous schist 

 5s more definite than where the series is most perfect; 

 it being impossible in the latter places to point out any 

 limit where the one commences or the other terminates. 



This general account of the argillaceous series-, as it 

 was required to illustrate the structure both of Arran 

 and of Bute, will also suffice for the description of that 

 portion of the latter island where it occurs. It is only 

 requisite to add that it is here displayed in great per- 

 fection, containing every one of the substances that have 

 been enumerated. The boundary will be better under- 

 stood by inspecting the map than by a verbal descrip- 

 tion; and it is sufficient to remark in addition, that 

 although definite near the sandstone, it is undefinable 

 near the micaceous schist, as it is every where on the 

 adjoining mainland. 



At a certain distance from this indefinite boundary, 

 the series of micaceous schist in this island becomes 

 exclusive ; appearing with the same characters which 

 it possesses throughout the adjoining district of Argyll- 

 shire and in many other parts of the mainland where 

 it approximates to the argillaceous series. It is so fre- 

 quently of a greenish colour as to render it doubtful 

 whether it should not be ranked with chlorite schist; 

 into which it here occasionally passes, as in the neigh- 

 bouring districts. The limits of those two rocks are 

 indeed so evanescent that it appears impossible to sepa- 



