2-J8 Dr. Mac Ciilloch on the 



sequently been demolished, showed the same tendency to exfo- 

 liation on all the exposed surfaces, into whatever forms they 

 might have been broken, and whatever their size might be. 

 Thus, in the smaller fragments, the process had already been 

 carried so far as to have solid balls covered with a succession of 

 crusts easily detached ; while, in the larger, it was still going 

 on, and promising ultimately to proceed to the same termination. 

 It is plain that, in the case of irregular fragments so formed, no 

 concretionary structure could be suspected ; as it is not within 

 the limits of possibility that they should have been broken from 

 the larger masses in such a fortuitous manner as that the 

 centre of the fragment should have coincided with a concre- 

 tionary centre. 



It must therefore be concluded, as enumerated at the com- 

 mencement of this division of the subject, that in trap as well as 

 in granite, two causes may act in producing the spheroidal de- 

 squamation, namely, an internal correspondent structure, aided 

 probably by the action of the atmospliere, and the operation of 

 that agent itself on the exposed surfaces of the same rocks, inde- 

 pendent of any such original arrangement. 



Having in the former part of this paper examined the nature 

 of schistose or exfoliating granite, and attempted, although 

 fruitlessly, to determine to which of the two cases under con- 

 sideration this effect was to be attributed, it will render this 

 subject more complete, to extend this inquiry to the case of 

 schistose, or straight laminar trap rock. If, in this case, also> 

 it appears as yet impossible to assign in all cases the true cause, 

 the investigation will not be without use. No distinct account 

 has yet been given in geological writings of this curious and 

 important circumstance in the history of the trap rocks, nor have 

 the various forms under which it appears been described; while 

 it is also certain that some mistaken views have been entertained 

 respecting it, as if it was limited to one species of trap in which 

 it formed an essential character, and which has been very 

 Improperly distinguished by this casual and merely incidental 

 circumstance, so as to be creeled into a species under the name 

 of porphyry slate. A more accurate description of these ap- 



