256 ON THE DESTRUCTION OF ROCKS. 



in producing the desquarnation of granite ; and as, in 

 cuboidal blocks, it cannot be determined to which of 

 these the effect must be assigned, the same difficulty 

 often occurs in those cases where the desquarnation 

 takes place in straight laminae. This is the case of 

 schistose granite, as it has been termed. 



The island of Arran affords very accessible exam- 

 ples of this, and it is Disposed, most generally, in 

 extended laminae of large dimensions, but is also occa- 

 sionally prismatic. The forcible fracture of these 

 blocks does not detect the slightest indication of a 

 laminar structure ; and there is no foliated disposition 

 in the integrant parts of the rock, which can account 

 for this desquamation ; as it does not bear the slightest 

 resemblance, even to granitic gneiss. 



The detached laminae are not less tenacious than 

 artificial ones of the same thickness would be ; ap- 

 pearing indeed, in every respect, perfectly natural. 

 Neither do their surfaces exhibit any signs of decom- 

 position ; being, on the contrary, brilliant and clean, 

 as if cut by art. The same cleanness and freshness of 

 both the surfaces in contact, are found where the 

 laminae and the block are separated ; nor is there any 

 loose matter generated, nor any appearances of de- 

 composition in the plane which disjoins them. The 

 thickness of the laminae varies from one eighth to a 

 quarter of an inch, but they are seldom uniform, in 

 this respect, throughout. 



From the nature of the desquamating crusts, it 

 might be supposed that this case resembled that of 

 the columns ; but, in this granite, the desqnamation 

 takes place only on that surface which is parallel to 

 the chief planes of the great laminae of the rock, and 

 not on the sides of the prisms ; whence it is probable 



