Desquamation of certain Eocks. 251 



western islands of Scotland, that it is unnecessary to particu- 

 larize the instances, I have observed it in clink-stone and in 

 clay-stone, as well as in the various porphyries which have these 

 ingredients for a base, and in the syenites. 



In one or two instances it occurs in a clay-stone which has a 

 columnar form; and here, contrary to the case described as 

 found in Sky, the laminar exfoliation is at right angles to the 

 axes of the prisms. It is proper to remark, however, that in 

 the only examples which came under my notice, the formation 

 of the laminae was superficial; there being rarely more than the 

 indication of three or four divisions, and not above one or two 

 of these being so far separated as to admit of being removed. 

 In no one case of this, or of the following occurrence of a 

 laminar exfoliation of shapeless masses, did I observe that the 

 deeper parts of the rock were affected, or that this structure 

 could be inferred to pervade the whole. 



In the above-mentioned cases also, where the exfoliation took 

 place at right angles to the axes of the columnar claystones 

 and porphyries, it is proper to remark that these were the only 

 surfaces exposed. Yet it is not improbable, that future ob- 

 servers will find that the sides as well as the ends of such 

 columns may present this appearance, to whatever cause it may 

 be owing, whether to simple exposure or to an internal corre- 

 sponding structure. 



As the rocks now enumerated are much more commonly found 

 in an irregular than in a columnar form, so the instances of 

 schistose exfoliation are sufficiently frequent in these to admit 

 of iheir examination in almost all situations where rocks of this 

 nature occur. 



The only one of these rocks which presents a large laminar 

 disposition analogous to that of granite, is hypersthene rock. 

 In this, the exfoliation in question sometimes occurs on the 

 surface ; but it is always limited to one scale, which, in its 

 thickness and tenacity, and in the general appearance at the 

 plane of separation, exactly resembles that which occurs both 

 in the schistose granite of Arran, and in the columns from 

 Leptis, already so often mcntioni-d. 



