244 Dr. Mac Culloch on the 



parallel, as already observed, to the great laminar structure ; and 

 lience it might be concluded, that both originated in the same 

 cause, or that the great concretionary structure which formed 

 the large lamina, was also the cause which influenced the de- 

 squamation of the small one. In this state of doubt must this 

 subject at present remain. 



It has, however, been sufficiently proved, that the process of 

 desquamation in granite does arise from two causes; but to 

 which of those two the case of the schistose granite is to be re- 

 ferred, must be a subject for future investigation. The facts are 

 in themselves singular ; the most remarkable have never yet 

 been noticed, nor have those which were formerly known been 

 examined with the attention they deserve. They have carelessly 

 been referred to one cause, and the present discussion will have 

 its use, even though it has not cleared up the difficulties of the 

 subject, by turning the attention of geologists to one among 

 many of those more recondite points in the history of rocks, 

 from the study of which geology has already derived, and is 

 probably destined to derive, much light. 



I shall now proceed to examine some cases in the rocks of the 

 trap family, which are of a similar nature, and which have 

 been equally neglected; as in these also the appearances in 

 question have invariably been referred to that one cause already 

 shown to be far from universal, namely, a peculiar concretionary 

 structure. It will be seen, that although some of the examples 

 of this occurrence in the trap rocks, are truly dependant on this 

 cause, 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 analogy to add to the numerous resemblances 

 which exist between granite and the rocks of the trap family. 



I need not call to the reader's attention the cases in which 

 columnar traps, whether basalt or green-stone, are known to 

 desquamate, as they are innumerable, and as instances of them 

 cannot fail to be known to all those who have any practical 

 acquaintance with the rocks of this family. In these cases, the 



