THE 



QUARTERLY JOURNAL, 



July, 1822. 



Art. f . On the Desquamation of certain Rocks, and on 

 its Connexion with the concretionary Structure. By 

 J. Mac Culloch, M.D., F.R.S. 



It is well known that many rocks of the trap family undergo 

 a process of desquamation after a long exposure to air, and are 

 thus gradually resolved into crusts which continue to fall off in 

 succession, at length mouldering into clay. The same appear- 

 ance, although more rarely, occurs also in granite ; and in both 

 cases, it has been conceived to depend on an internal concre- 

 tionary structure, and to indicate the mode in which the consti- 

 tuent parts of the rock are arranged, which, however invisible 

 in the fresh fracture, is thus rendered evident by the progress of 

 decomposition. In the case of the columnar traps, whether 

 basalt or greenstone, this desquamation often proceeds in 

 such a manner from the circumference of a joint towards the 

 centre, that the ultimate result is a spheroidal body. 



This eflect, compared with that spheroidal concretionary struc- 

 ture which is known to take place in basalt artificially fused, has 

 appeared sufficient to justify the general conclusion, that all 

 appearances of a similar nature depend on the same cause ; 

 and, by a slight addition, it has been held sufficient to account 

 for the jointed and columnar structure of the rock in which it 

 occurs. Having met with some remarkable instances in which 

 this effect was decidedly independent of any concretionary 

 structure, I was induced to examine more particularly the 

 nature of the evidence which had been held suflicient to prove 



Vol. XUI. S 



