Mr Thomson 07i the Farallel Roads of Lochaber. 59 



cial theory I have maintained. The same mass of ice occu- 

 pying Loch Lochy, which I have supposed to have been in- 

 strumental in forming the shelf in Glen Gluoy, would, to all 

 appearance necessarily, have blocked up also the glen at Kil- 

 finnan, and thus have produced the shelf which is really 

 found to exist round its sides, on a level with the water-shed 

 at its top. Mr Milne himself mentions the occurrence, in 

 various parts of the Highlands, of other glens containing 

 shelves, none of which have, however, been so carefully in- 

 vestigated as those we have been considering. According 

 to what I have already said, this would appear to add to the 

 difficulties of the explanation by means of eai'thy banners, 

 and to confii'm the one I have given, depending on the agency 

 of a climate such as would cover with a thick bed of ice almost 

 the whole surface of the land in the neighbourhood of high 

 mountains. 



It will be unnecessary for me to enter at length into a dis- 

 cussion of the diluvial -theory of the parallel roads, given by 

 Sir George Mackenzie, as, after a full consideration of it, it 

 does not seem to me to be capable of explaining the observed 

 facts. I may, however, mention some of the leading objec- 

 tions which I would bring against it. During the sinking 

 of the supposed wave, on the arrival of its surface at each 

 successive summit-level, there would be no sudden check to 

 the flow of the water through the glens, nor even to the rate 

 of depression of the general surface of the wave ; but even 

 if some material alteration in the flow of the water were to 

 occur at those particular occasions, there seems to be no 

 reason to suppose that these vast shelves would be the result. 

 No attempt, besides, is made according to this theory, to shew 

 why the various shelves should be expected to stop short at 

 the particular places where, by observation, they are found 

 to do so. 



An objection which has been urged by Mr LycU against 

 the glacial theory of the parallel roads must not be left un- 

 noticed. He thinks there are proofs to be met with in va- 

 rious parts of Scotland of great changes having occurred in 

 the relative levels of the eea and land ; and he supposes that 

 such changes would have destroyed the horizontality which 



