258 ON THE DESTRUCTION OF ROCKS. 



mately resolved into loose earth. But these changes 

 are not limited to prismatic trap only, since they 

 occur in other and different forms ; being attended 

 with the same final result, whatever the shape of the 

 block may have been, namely, the ultimate production 

 of spheroids. 



Although, in some of these instances, it might be 

 imagined that the effect of desquamation was pro- 

 duced, as in the case of the artificial granite columns, 

 by the exposure of the surfaces to the air, it will, I 

 believe, be found that, in nearly all, they truly depend 

 on an internal concretionary structure. 



The chief argument for this opinion will be found 

 to consist in the effects which take place in those traps 

 that are not jointed, as well as in those masses which 

 affect a prismatic fracture without being absolutely 

 divided into prismatic forms. In these, as in some 

 of the cases of exfoliating granite, the desquamation is 

 of a complicated nature, referring to more than one 

 centre. Thus, in a single unjointed prism, the same 

 result takes place as in those with joints ; numerous 

 spheroids being discovered in its length, resulting from 

 the progress of desquamation. The same effect is 

 produced in those irregular masses which are charac- 

 terized merely by a prismatic fracture ; as the exfolia- 

 tion commences in many different places, referable to 

 different central points, so as to leave, in the same way, 

 a number of spheroidal bodies imbedded in a mass of 

 loose crusts and clay. It is further, indeed, often to 

 be observed, that in cuboidal or otherwise irregular 

 blocks which have neither prismatic form nor ten- 

 dency, there are several centres of exfoliation : nu- 

 merous balls being thus finally extricated from a single 

 solid block, of which all the surfaces are equally 

 exposed to the action or contact of the atmosphere. 



