ON THE DESTRUCTION OF ROCKS. 259 



It must now be observed on the other hand, that 

 masses and fragments of trap, which have received 

 their forms by art or accident, sometimes show the 

 same tendency to exfoliation on all the exposed sur- 

 faces ; into whatever forms they may have been 

 broken, and whatever their size may be. In the 

 smaller fragments, the process has sometimes been 

 carried so far as to leave solid balls covered with a 

 succession of crusts easily detached. It is plain that, 

 in the case of irregular fragments so formed, no con- 

 cretionary structure can be suspected ; as it is not 

 within the limits of possibility that they should have 

 been broken from the larger masses, in a fortuitous 

 manner, and so that the centre of the fragments 

 should have coincided with a concretionary centre. 



It will render this subject more complete, to extend 

 this inquiry to the case of the schistose, or laminar 

 trap ; a subject which has either been neglected or 

 much misapprehended. 



The tendency to flat laminar exfoliation on the 

 surfaces, is more common in the rocks of this exten- 

 sive family, than the spheroidal, since it occurs in 

 every species that I have examined; whereas the latter 

 is rarely found except in basalts and greenstones. The 

 internal flat laminar structure which is independent of 

 the agency of the atmosphere, seems also to exist in 

 a greater number of species than the spheroidal con- 

 cretionary form does. 



This species of exfoliation of the surface of trap, 

 occurs in so many parts of the Western Islands of 

 Scotland, that it is unnecessary to particularize the 

 instances. In one or two, it is found in a claystone 

 of a columnar form ; the exfoliation being at right 

 angles to the axis of the prism. There are rarely 



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