ON THE DESTRUCTION OF ROCKS. 253 



Of the Desquamation of Rocks. 



It is well known that many rocks of the trap fami- 

 ly, 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 appearance, 

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

 both cases, it has been conceived to depend on an in- 

 ternal concretionary structure, and to indicate the 

 mode in which the constituent parts of the rock are 

 arranged, which, however invisible in the fresh frac- 

 ture, is thus rendered evident by the progress of de- 

 composition. In the case of the columnar traps, 

 whether basalt or greenstone, this desquamation of- 

 ten proceeds in such a manner, from the circumference 

 of a joint towards the centre, that the result is a sphe- 

 roidal body. 



This effect, compared with the spheroidal concre- 

 tionary structure which is known to take place in 

 basalt artificially fused, has appeared sufficient to justi- 

 fy 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 also to ac- 

 count for the jointed and columnar structure of the 

 rock in which it occurs. The result of a more careful 

 examination has been to prove, that two causes, per- 

 fectly distinct from each other, operate in producing 

 the same effects; and a detail of the facts in question 

 will not only serve as a caution against the universal 

 adoption of a well-known rule in philosophizing, but 

 to record an interesting circumstance in the history ot 

 the rocks in which these appearances have been 

 observed. 



