Desquamation of certain Rocks. 245 



crusts fall off in succession, in such a manner that the angular 

 or prismatic form at length disappears ; and the ultimate result 

 is therefore a spheroidal body, destined ultimately to be also 

 resolved into clay. It most frequently happens, that as the 

 crusts are separated they also fall to pieces, or become loose 

 clay; but occasionally they retain a considerable degree of 

 tenacity, although from the changes of their colour from the 

 natural dark blue to brown, i^ is evident that the iron has 

 undergone a chemical alteration, being converted from protoxide 

 to rust. 



Where similar rocks possess a rude and imperfect prismatic 

 structure, the same effects also take place ; the surfaces of the 

 prisms desquamating in such a manner as to leave a congeries 

 of spheroidal or ellipsoidal bodies, destined, in the same way, 

 to be ultimately 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. In 

 some rare instances of this nature, it is remarkable that a loose 

 spheroid is sometimes found, on fracture, to be imbedded in an 

 irregular mass apparently solid, or at least giving no exterior 

 signs of desquamation, and bearing no other marks of change 

 than the loss of the blue and the assumption of the brown 

 colour. 



Although, in some of these instances, it might be imagined 

 that the effect of desquamation was produced, 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. In the 

 case of aggregated columns, for example, the contact of the 

 prism is generally so complete that neither water nor air can 

 gain access to the surfaces. 



The same rule sometimes holds with respect to the joints ; 

 although, in many of these, perhaps in most, the contact is not 

 so perfect as to exclude either air or water, as is proved by the 

 changes of colour which their surfaces exhibit. But the chief 



