380 Dr. Mac Culloch on the Parallel Roads of Glen Roy> 



these different drainages as having occurred at different barriers. 

 Admitting then that the corroding action of the waters of the Spean 

 and Roy operating on an alluvium at the exit of the Lochy, had, 

 by destroying a portion of the barrier, discharged that portion of 

 the lake which stood above the second line of Glen Roy, a vertical 

 distance of 82 feet, we have still left standing the other barriers, 

 of the existence of which we cannot doubt, although their place 

 cannot be precisely assigned. By what operation then were these 

 lowered ? If by any causes of a nature similar to those which we 

 see in daily action on the surface of the earth, it must have been by 

 the flowing of rivers upon them. Thus the flow of the Ness and the 

 Spey towards the sea, might have lowered the land in these di- 

 rections to their present level, and thus the exit of Loch Shiel might 

 have destroyed the barrier to the west ; while the repeated failures 

 of the supposed barrier at the mouth of the Lochy had in the mean 

 time produced the complete drainage of Glen Roy and Glen Gloy, 

 and with the exception of Loch Laggan, that of the Spean. 



I know not that the direct arguments which have been here stated 

 are sufficient to prove that hypothesis respecting the lines of Glen 

 Roy which appears to be the best founded ; or whether, combined 

 with those indirect ones which prove the impossibility of two of the 

 others and the high improbability of the third, they may be held 

 sufficient to establish its truth. I have however shown, that although 

 it still labours under unexplained difficulties, no physical impossibility 

 is in any way opposed to its superior probability. We may there- 

 fore admit its claim for the present, at least so far as to justify us in 

 examining the geological consequences likely to result from it. 



I must in the first place remark, that the causes here assigned for 

 the appearances in Glen Roy are attended by consequences materi- 



