ON THE DESTRUCTION OF ROCKS. 255 



tion those cases in nature where the same process 

 takes place on the exposed surfaces, it will be found 

 that the desquamation of the prismatic or cuboidal 

 masses of granite which are susceptible of this change, 

 takes place all round the surface, respecting some 

 imaginary point or centre, and promising, in the pro- 

 gress of time, to reduce the whole to a smaller and more 

 spheroidal mass. Hence no conclusion can be drawn 

 as to the cause; since the desquamation may, in this 

 case, be either the result of an internal concretionary 

 and laminar structure respecting one centre, or the 

 consequence of a process similar to that occurring in 

 the columns of Leptis. 



But in examining other cases of desquamation in 

 granite, a different appearance will be observed. In 

 these, it may be seen that a single block desquamates 

 in a manner so complicated that no parallelism is main- 

 tained between the surfaces of the stones and the 

 crusts; and as, in some cases, such blocks are so 

 thoroughly softened as to admit of being cut by a 

 spade, it is not difficult to discover that more than 

 one, or even two centres of desquamation, exist in a 

 single mass; the surfaces of the different spheroids 

 interfering and compressing each other where they 

 come into contact; and the intermediate parts, which 

 are still required to fill up the solid, consisting of 

 deficient portions of crusts, respecting one or other 

 of the approximate, imbedded or internal spheroids. 

 It is evident that, in these instances, the effect could 

 not have resulted from the action which produced 

 the crusts on the columns just described; but that it 

 must have been determined by other causes, depending 

 on an interior structure, the existence of which has 

 already been proved. Thus, two distinct causes act 



