Desquamation of certain Rocks. 255 



schist of a foliated and somewhat undulated structure, and being 

 also characterized by a considerable quantity of quartz propor- 

 tioned to the mica which enters into its composition. It is a 

 very hard variety of this rock; the quartzose laminae being 

 compact and of considerable thickness. 



From the universally weathered state of all the exposed sur- 

 faces, and from the succession of lichens which appear to have 

 grown and decayed on it, a long period must have passed since 

 it quitted the mountain above : if ever applied to the objects of 

 Druidical worship, as is here supposed, we can at any rate 

 assign pretty nearly the minimum of time during which it has 

 lain where it now stands. 



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 of the mica, or in its 

 assuming a brown hue, it is only on one side that the peculiar 

 effect which forms the object of this paper has taken place. 

 This is the north-eastern, or northern quarter; and, from the 

 form of the valley, it is probably that on which the wind and 

 rain beat with most violence. This circumstance furnishes a 

 proof, if proof were wanting, that the effect, however inex- 

 plicahle, is the result of atmospheric action. 



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 have ex_ 

 foliated from the granite columns described in the former paper. 

 Their thickness varies in different parts, from the sixth of an 

 inch to nearly the half; and they may be obtained, by careful 

 application of a knife or thin chisel, in plates containing from 

 six or eight to twelve or sixteen superficial inches. At the 

 planes where they separate from the mass of the rock, no 

 powdery matter, or grains of sand, or rusty clay, or any other 

 marks of real decomposition are visible ; both the surfaces being 

 clean and smooth, as if cut by a sharp tool. 



It must further be observed, that there is here a succession 

 of similar operations visible. At some former period, the 

 jjlalcs first detached have in some places fallen off, so as to 

 T2 



