352 Dr. Mac Culloch on the Parallel Roads of Glen Roy. 



failure of some barrier by wh'ich they had been confined, it is plain 

 that considerable changes of the surface are requisite for the so- 

 lution of the present appearances, as well on this supposition as on 

 either of the former ones, although these changes must be of a dif- 

 ferent nature. 



Each of these hypotheses is attended with considerable difficulty, 

 and involves consequences as important in a geological view as 

 they are unknown in our ordinary experience. Arguments enough 

 have perhaps been brought to show that they could not have been 

 works of art ; and among the natural causes which present them- 

 selves I know not that any others can be produced but the three 

 now mentioned. It is our duty therefore to examine the pro- 

 babilities attached to each of these, and to chuse among them that 

 whose ordinary effects offer the fewest discrepancies from the actual 

 appearances under review. If the whole of the phenomena are 

 still difficult of explanation under any system which we may 

 adopt, we must have recourse to the method of dilemma, and at 

 least reject those assigned causes which involve impossibilities. If 

 it shall finally appear that an impossibility is attached to each, we 

 shall be driven back to allow their origin in human art and labour ; 

 since this hypothesis involves at least no physical impossibility, 

 though assuredly a very high degree of moral and physical im- 

 probability. 



The first hypothesis which has been proposed to explain the 

 appearances in Glen Roy is the action of a deluge, or rather of a 

 series of large and powerful torrents. There must in fact have 

 existed three torrents at distinct periods, as the nature and distances 

 of the several lines obviously require such a series of causes. It is 

 neither necessary nor convenient to examine the general principle 



