ON THE DESTRUCTION OF ROCKS. 257 



that the great concretionary structure which formed 

 the large laminae, is also the cause which influenced 

 the desquamation of the small one. 



Of the Desquamation of Trap. 



It will next be seen, that although some of the 

 examples of this occurrence in the trap rocks, are 

 truly dependent on the concretionary structure, others 

 are as unquestionably the result of actions similar to 

 those which produce the desquamation in artificial 

 blocks of granite. Thus they confirm the views, 

 already held out, of one common effect proceeding 

 from two causes ; while they also offer another ana- 

 logy, to add to the numerous resemblances which 

 exist between granite and the rocks of the Trap 

 family. 



In the columnar traps, whether basalt or green- 

 stone, the 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 finally to be also resolved 

 into clay. It most frequently happens, that as the 

 crusts are separated, they also fall to pieces, or be- 

 come loose clay ; but, occasionally, they retain a 

 considerable degree of tenacity ; although, by the 

 changes of their colour from the natural dark blue to 

 brown, it is evident that the iron has undergone a 

 chemical alteration ; being converted from protoxyde 

 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 ellip- 

 soidal bodies, destined, in the same way, to be ulti- 



VOL. i. s 



