Desquamation of certain Rocks. ' 255 



below the exfoliating surfaces, will be found to present the same 

 appearance as the detaclied scales. But if a deeper section be 

 made, it will, I believe, invariably be found that the natural 

 state of the rock is different, and that the process of exfoliation 

 has been attended by a partial decomposition or change. 



I may here briefly remark, while on this subject, that the 

 term schistose porphyry, already roticed as applied to the rocks 

 of that character which are susceptible of this change, is inju- 

 dicious. Such a term might, even with more propriety, be 

 applied to the laminar porphyries with a base of compact 

 feldspar that occur in veins ; since in them it appears more truly 

 a natural conformation, and independent of the action of ex- 

 traneous or accidental causes. But in any case it is improper 

 to erect a species from distinction^ which are merely of an 

 accidental nature, and which, if in this case they are not abso- 

 lutely produced, are still materially influenced by accidental 

 causes. Nor is there any reason why the term schistose should 

 be limited to the porphyries alone that present this character ; 

 as there is the same reason for extending it to the olay-stones 

 and syenites that exhibit the same appearances. I have here 

 used the term schistose porphyry in making these animadver- 

 sions. That of porphyry slate, commonly in use, is still more 

 objectionable; but 1 need not prolong these remarks, having 

 already noticed this circumstance in my Account of the Western 

 Islands. 



It only remains, in concluding this subject, to inquire whe- 

 ther the exfoliation of the laminse, in these last examples, 

 depends, as in the case of artificial blocks of trap and granite 

 already described, upon the action of the atmosphere alone, or 

 whether it is the consequence of this action coinciding with an 

 irfiernal and original concretionary arrangement. On this sub- 

 ject, it appears to myself that the arguments on each side are 

 pretty nearly balanced. A similar structure of a concretionary 

 nature is proved to exist in some traps, and therefore it may be 

 conceived to exist in these also. On the other hand, it takes 

 place only within the influence of the atmosphere, and is 

 attended with a degree of decomposition which indicates the 



Vol. XIH. T 



