Desquamation of certain Rocks. 239 



It is worthy of remark, that, in this case, the detached crust 

 is not decomposed, and that, except in tenderness and fragility, 

 it appears scarcely changed from its natural state. Nor is 

 any stratum of clay or decomposed matter found at the place 

 where the crust separates from the solid block ; at least none 

 such was found in the places which I had an opportunity of 

 examining. The thickness of the crust appeared to be between 

 the sixth and the eighth of an inch ; but it seemed unnecessary 

 to make any accurate measurement of that which is accessible, 

 perhaps to all who may be interested in the subject of this 

 paper. 



The decomposition of rocks has, in most cases, and with 

 justice, been attributed to changes in the state of the iron enter- 

 ing into their composition ; but it is evident, that neither this, 

 nor the other causes which have been supposed to produce that 

 effect, are capable of explaining the very singular process in 

 question. As it is in vain to speculate on that to which there 

 is no known analogy, I shall not attempt it. It must, never- 

 theless, be concluded, that this effect cannot arise from that 

 action of the atmosphere which produces the ordinary effect of 

 weathering ; nor is it conceivable how it should affect the inte- 

 rior part of the stone, while the surface more immediately ex- 

 posed to its agency has escaped. 



Being in possession of this singular fact, I was induced, as 

 before remarked, to suspect the common opinion respecting the 

 cause of the desquamation of granite, and was therefore led to 

 re-examine with more attention those causes in nature where 

 the same process takes place on the exposed surfaces. 



In the greater numbers of these, I only found that which has 

 been observed by every one, namely, that the desquamation of 

 the prismatic or cuboidal masses of granite which are suscep- 

 tible of this change, took place all round the surface, respect- 

 ing some imaginary point or centre, and promising, in the pro- 

 gress of time, to reduce the whole to a smaller and more sphe- 

 roidal mass, llcnce, no conclusion could be drawn as to the 

 cause ; since the desquamation might in this case be the result 

 of an iutornul concretionary and laminar structure respecting 

 S 2 



