264 Dr. Mac Culloch on a 



from the facility with which this rock is often decomposed and 

 removed, and the great destruction of substances here visible, 

 it is not improbable that some mass of it has once been in 

 contact with the columnar part of the bed of shale which forms 

 the ironstone under review. 



In describing the country about Dunbar, I shewed that 

 detached superincumbent masses of trap occurred in various 

 places, and that, in many, they gave striking marks of rapid 

 decomposition. The ultimate removal of such masses is 

 proved by their gradual evanescence in other parts of Scotland ; 

 in many districts of which they may be traced gradually di- 

 minishing in size as they recede from some larger mass, till 

 they vanish, leaving proofs of their former existence in this 

 peculiar nature of a soil, which those who have once been 

 intimate with it will never fail to recognize. The conclusion 

 to be drawn from this reasoning is, that one of these masses of 

 trap has originally covered the prismatic sandstone of Dunbar, 

 and that to the influence of this is owing the change both of 

 structure and mineral character which it has undergone. 



In terminating these remarks, I do not feel it necessary to 

 account for exceptions ; for it may be said, that this effect, if 

 depending on that cause, ought to occur more frequently. 

 The common answer to this often-repeated objection against 

 all general conclusions from particular facts is sufficient. 

 Nothpg can be pronounced an exception till all the conco- 

 mitant circumstances, or those capable of modifying the pre- 

 sumed results, are known. 



Of these we know little at any time ; and the objection, 

 such as it is, applies equally to every case where the influence 

 of trap on the neighbouring rocks is visible. 



It now remains to inquire how far the preceding facts and 

 reasonings can be applied to the often-discussed question of 

 the prismatic configuration of the trap rocks. Here we have, 

 at least, some strong analogies to guide us. It is certain that, 

 after such fluid masses had become solid, they must, for a long 

 period, have retained a considerable degree of heat, and under- 

 gone a process of slow cooling. It has, indeed, been supposed 

 by many, that the prismatic configuration in the traps has 

 taken place during the act of consolidation, or that it was a 



