Desquamation of certain Rocks. 243 



of the columns of Leptis, it arises from some unknown effect of 

 the weather on the surfaces of rocks possessing no original 

 peculiar structure of this nature. 



If we were to form a judgment from merely comparing the 

 appearances in the case of the schistose granite, and in that of 

 the columns, we should be inclined to refer them both to the same 

 mysterious cause. The appearances in both cases are precisely 

 similar, and, in both, they differ from that case in which the 

 desquamation is demonstrated to be the result of an internal 

 concretionary structure; since, in this, it invariably happens 

 that the scales are not only separated, but decomposed, losing 

 their integrity, and falling to powder and gravel on the slightest 

 injury. In this case also, the desquamation and decomposition 

 affect many successive lamina, proceeding in some cases, 

 indeed, even to the central point, or to the heart of the block : 

 whereas, in the case of the schistose granite, as in that of the 

 artificial columns, only one lamina, or a minute space beneath 

 the immediate surface, is affected. 



Those who are in haste to establish laws and complete theo- 

 ries, little concerned whether or not they are well founded, will 

 be content to repose in this conclusion. But those, to whom 

 truth has a value superior to all other considerations, will prefer 

 doubt to decision, while any ground for hesitation remains. 

 It has already been stated, that in the columns at the British 

 Museum, the desquamation took place over the whole surface, 

 so as to have produced, could it have been completed all round, 

 a hollow cylinder. The same occurs in the decomposition of 

 those natural blocks where the laminaj refer to one centre only. 

 But in the case of the schistose granite, it must be observed, that 

 the desquamation only took place on one surface, and that sur- 

 face parallel to the chief planes of the great lamina; in which the 

 granite is disposed. No desquamation at right angles to this 

 was observed ; or while the larger planes gave off scales, the 

 narrower lateral ones were free from them. This should not 

 have happened had the effect arisen from the same cause, as in 

 the artificial columns; but every exposed suiface should have 

 desquamated alike. On the contrary, the desquamation was 



