260 ON THE DESTRUCTION OF ROCKS. 



more than the indications of three or four divisions ; 

 not above one or two of these being so far separated 

 as to admit of being removed, and the deeper parts of 

 the rock remaining unaffected, so that this structure 

 cannot be inferred to pervade the whole. As the 

 irregular rocks are much more common than the 

 columnar, the instances of schistose exfoliation are, in 

 these, similarly, more frequent. 



Among the different kinds, the only one which 

 presents a large laminar disposition analogous to that 

 of granite, is Hypersthene rock. In this, the exfolia- 

 tion sometimes occurs on the surface ; but it is always 

 limited to one scale or lamina, which, in its thickness 

 and tenacity, and in the general appearance at the 

 plane of separation, exactly resembles that which is 

 produced in the schistose granites. 



Few marks of such a large laminar disposition are 

 to be observed in the Syenites, claystones, and clink- 

 stones, or in the porphyries derived from the two 

 latter rocks ; though Mull presents some remarkable 

 exceptions. No relation can therefore be inferred 

 between the direction of the exfoliation and that of a 

 larger mass ; and accordingly, we can only conclude 

 that, whatever surface is thus found exfoliating, the 

 direction of this change relates to that of the exposed 

 surface. Yet as these rocks seldom exfoliate in two 

 directions at any one place, it is probable that this 

 tendency does actually bear some relation to the inter- 

 nal structure of a mass, though no other indications 

 of it are visible. That such a structure does occur in 

 many traps, is an argument in favour of this opinion. 



The number of successive laminae which may be 

 detached from any of the rocks last named, is various ; 

 but I know not that it bears any constant relation to 



