254 Dr. Mac Culloch on the 



influence of air and water on the rock. It seems more prudent 

 to leave this question undecided for future examination ; and I 

 shall therefore terminate this paper, satisfied with having placed 

 the facts in such a state as to be ready for any future illustration 

 which geologists may add to the subject. 



J. Mac Culloch. 



Additional Remarks on the Desquamation of Rocks. 



When the foregoing paper was written, I had not observed 

 the desquamation which is there described, in any rocks but 

 those of the families of the Granite and Trap. But an inte- 

 resting case of this nature having since occurred in one of the 

 primary stratified substances, it has appeared to me worthy of 

 being added as a supplement to that record. It is true that it 

 does not throw any additional light on the immediate cause, or 

 on the mode in which the atmosphere operates in producing tliis 

 remarkable effect. Yet it proves clearly, as clearly as the case 

 of the columns of Leptis, that this effect is not dependent on 

 any internal concretionary structure in the stone, and that it is 

 not even limited to those rocks which are so often characterized 

 by that peculiarity. 



In Glen Almond, a well-known valley, through which the 

 military branch of road from Stirling to Dalnacardoch passes, 

 and which conveys the river Almond from its native hills to the 

 great plain of Strathearn, there is an insulated stone long 

 known by the name of Ossian's Tomb. Whatever interest this 

 belief may excite in the breast of a true Highlander, a geologist 

 is very little concerned with respect to that visionary claim of 

 honour; but, from some indications of a circle of which it is 

 the centre, it appears rather to have been adopted as the centra^, 

 stone of a Druidical structure. From its weight and bulk, it 

 has unquestionably not been placed by art in its present posi- 

 tion; but appears, on the contrary, to be a block which, like 

 many others, has fallen from the impending precipices of the 

 hills above. 



It corresponds in its nature with these; being a micaceous 



