262 ON THE DESTRUCTION OF ROCKS. 



ception of the nature of this phenomenon, we are 

 indebted for the very improper term Porphyry slate, 

 and the much more improper distinction. 



Of the Desquamation of micaceous Schist. 



Whatever difficulties may appear to exist in ex- 

 plaining these appearances, they are exceeded by a 

 similar one in Micaceous schist ; although, but one 

 example of this, has occurred during my researches. 

 It is in the large block known by the name of 

 Ossian's tomb, which lies in Glen Almond. 



Although this stone bears, on all the surfaces, 

 those slight marks of decomposition so well known 

 in micaceous schist, which consists in the rusting of 

 the iron in the mica, it is only on one side that the 

 peculiar effect in question has taken place ; being 

 that on which the wind and rain beat with most vio- 

 lence. From nearly the whole of this surface, scales 

 of the rock can be detached ; scarcely differing in 

 tenacity and hardness from the original stone, and 

 thus resembling those which exfoliate from the gra- 

 nite columns. Their thickness varies in different 

 parts, from the sixth of an inch to nearly the half; 

 and they may be obtained in plates of considerable 

 extent. At the planes where they separate from the 

 mass of the rock, no marks of decomposition are 

 visible ; both the surfaces being clean and smooth, as 

 if cut by a sharp tool. 



There is here also a succession of similar operations 

 visible. At some former period, the plates first de- 

 tached have in some places, fallen off, so as to leave 

 a new surface exposed ; and, on this, the same de- 

 squamation has again taken place, so that, in some 



