failure of some barrier by which they had been confined, it is plain that considerable changes of the surface are requisite for the solution of the present appearances, as well on this supposition as on either of the former ones, although these changes must be of a different 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 themselves I know not that any others can be produced but the three now mentioned. It is our duty therefore to examine the probabilities 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 improbability.
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
