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 directions 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 therefore 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 materially
