Dr. Mac Culloch on the Parallel Roads of Glen Roy. 379 



on the borders of Loch Lochy, as well as on the principal extent 

 of the borders of Loch Laggan, and the valley of the Spean. There 

 is a set of common features through the whole tract, the same rocks, 

 the same slopes, the same causes of waste, yet the water marks are 

 strongly defined through a portion of this wide space, while they 

 are totally wanting in others. If we increase the extent of the 

 supposed lake, we increase this difficulty, but it is already sufficiently 

 great. It is proper to notice that this objection to the hypothesis 

 of a lake, is equally applicable to the other two modes offered for 

 explaining the action of water in producing these lines. If it is 

 held sufficient to reject the one, it is equally valid against the other 

 two, although it seemed superfluous to enumerate it among the more 

 serious difficulties to which those are liable. 



The complete and sudden transition from the uppermost line of 

 Glen Roy to the next succeeding one, and finally to the present 

 bottom of the valley or perfect draining of the whole, shows that the 

 lake which occupied these vallies had subsided at three different in- 

 tervals. As it is a more probable supposition that these three suc- 

 cessive drainings took place at the same point, than at different ones, 

 it will be most convenient to assume the present and lowest com- 

 munication, namely, the real exit of the waters of the Spean and 

 Roy, as that point. This place must be situated on the river 

 Lochy, before its junction with the sea at Loch Eil. Here then we 

 must imagine that a dam has subsisted, not gradually worn down by 

 the slow corroding action of the river issuing from the lake, but by 

 three successive failures occurring suddenly, or at least within short 

 intervals of time. Had much time elapsed between these intervals, 

 the several lines must have been more obscurely marked, or interme- 

 diate ones of smaller dimensions must have been visible. It is plain 

 that the difficulties will be unnecessarily complicated if we consider 



