246 Dr. Mac Culloch on the 



argument for an internal structure as the cause of the desqua- 

 mation of prisms, will be found to consist in the effects which 

 take place in those that are not joined, as well as in those masses 

 of trap which affect a prismatic fracture without being abso- 

 lutely divided into prismatic forms. 



In these it may be observed, as in some of the cases of exfo- 

 liating granite formerly cited, that the desquamation is of a 

 complicated nature, referring to more than one centre. Thus, 

 in a single unjointed prism, the same result takes place as in 

 those with joints ; numerous spheroids being formed in its 

 length by the process of desquamation. The same efl'cct is 

 produced in those irregular masses that are merely characterized 

 by a prismatic fracture, as the exfoliation commences in many 

 different places, referable to different points, so as to leave, in the 

 same way, a number of spheroidal bodies imbedded in a mass 

 of loose crusts and clay. 



Indeed it is further often to be observed, that, in cuboidal or 

 otherwise irregular blocks which have neither prismatic form 

 nor tendency, there are several centres of exfoliation; numerous 

 balls being then finally extricated from a single solid block, of 

 which all the surfaces are equally exposed to the action or con 

 tact of the atmosphere. 



It appears therefore unquestionable, that in all the instances 

 now enumerated, where the exfoliations respect one or more 

 centres, and the detached crusts are therefore of spheroidal 

 forms, the original cause consists in a spheroidal concretionary 

 structure in the rock, of a nature more or less complicated, 

 although we are equally far from being able to explain in what 

 manner the action of the elements operates on that structure so 

 as to produce the peculiar effects in question. In terminating 

 this part of the subject, I shall quote an analogy in favour of the 

 existence of tliis structure in prismatic traps, which being, in 

 other respects, of an interesting nature, I purpose to give a more 

 full account of it at some future period. 



In this case, which is found in an argillaceous sand-stone, a 

 large bed of that rock is in some places not only marked by a 

 tendency to prismatic division at right angles to the plane of 



