L'38 Dr. Mac CuUncli on llic 



the current explanation ; lest, having been proved inadequate to 

 the solution of many cases of this nature, it might turn out purely 

 hypothetical and inapplicable to any. The result has been, 

 that two causes, perfectly distinct from each other, operate in 

 producing the same efl'ect ; and it will be useful to state the facts 

 in question, not only as a caution against the universal adop- 

 tion of a well known rule in philosophizing, and against a hasty 

 mode of generalization, too often adopted in geological science, 

 but as forming an interesting circumstance in the history of the 

 unstratified rocks, trap, and granite. 



I was first led to suspect the truth of the current opinion 

 respecting the dependence of the desquamation of rocks on an 

 internal concretionary structure, by casually examining the 

 columns of granite lately brought from Leptis in Africa, and now 

 lying in the great court of the British Museum. I was sur- 

 prised to find that the shafts of these columns were in the act of 

 desquamation, casting off crusts precisely similar to those 

 which occur in many cases in natural blocks of granite, and 

 equally resembling, except in their superior integrity, those that 

 are found on tlie surfaces of the columnar trap rocks after ex- 

 posure to the weather. As it was obvious that the forms of the 

 shafts could bear no relation to the original form of the blocks 

 of granite, from which they had been wrought, it necessarily fol- 

 lowed, that in this case at least, the desquamation could not 

 depend on an internal concretionary structure, but must have 

 resulted from the action of the weather on the exposed sur- 

 faces. 



These specimens being fortunately accessible to many of our 

 geological readers, they will be enabled to confirm, by their 

 own observation, this important fact ; nor can the report un- 

 dergo the hazard of being suspected of inaccuracy or mis-state- 

 ments ; an expedient to which recourse is often had for the 

 purpose of obviating a diflficulty, or eluding a fact which mili- 

 tates against some favourite hypothesis, when such a fact is 

 distant or difficult of access ; a proceeding which is indeed un- 

 fortunately but too much justified by the numerous inaccurate 

 statements in which geological writings abound. 



