Desquamation of certain Rocks. 241 



sents. Those who may have an opportunity, and who are de- 

 sirous of examining and verifying this statement, will find it 

 situated in the upper part of Glen Catcol, nearly at the very rise 

 of the stream which flows through that valley. 



This granite is of a fine texture, and consists principally of a 

 pale feldspar, intermixed with a moderate proportion of quartz; 

 small crystals of hornblende being sparingly interspersed. It is 

 disposed most generally in extended laminse of large diraeu- 

 sions, but is also occasionally prismatic ; the prisms being how- 

 ever very irregular, and commonly of a small size. 



The schistose specimens, or the examples of desquamation, 

 which are here the principal objects of notice, do not occur in all 

 the granite at this place ; nor is there any apparent cause for its 

 capricious occurrence, as it is sometimes present, and at odiers 

 wanting, where the eye does not trace any differences in the ap- 

 pearance or composition of the mass. 



It does not often happen that more than one laminse can be 

 detached from a block ; and when removed, no further indica- 

 tion of a similar tendency is in general to be discovered. Neither 

 does the forcible fracture of these blocks detect the slightest 

 indication of a laminar structure; the whole appearing as uni- 

 form as any granite which is not marked by this remarkable 

 character. It must also be observed, that there is no disposition 

 in integrant parts of the granite which can account for this de- 

 squamation. It does not bear the slightest resemblance even to 

 those specimens of gneiss which approach most nearly to the 

 granite character, nor is there the least indication of a foliated 

 disposition in the integrant minerals ; the whole texture being as 

 uniform in the detached laminse as in the block, and being in- 

 deed peculiarly compact. 



Although two schists, or laminae, cannot be detached in suc- 

 cession from one surface, there is sometimes the commence- 

 ment of a second, indicated by a line on the exterior side, into 

 which the blade of a knife can be insinuated for a small space. 

 But it is more common to find this indication alone ; the lamiujie 

 previously formed, having fallen off and been washed away 



