The case of Glen Spean presents other difficulties of its own; but we can suffer our readers to make the neces sary application of the reasonings we have thus furnish ed. That of Glen Glop is not less so. No current could flow from Glen Roy through it on account of the height of the bottom of Glen Turit already mentioned. None could have originated in it, nor could one have flowed from its entrance at Lowbridge towards the east, where the waters could have found no exit. We hold it quite unnecessary to pursue this unfounded hypothesis further, and shall therefore proceed to examine that proposed by Lord Selkirk.
as those which now occupy the bottom of the valley. This explanation will be found to offer a solution for all the most remarkable of these cases, and very particularly for those at the entrance of Glen Turit.
It must not here be overlooked, that their correspondence in altitude on the opposite side of the glen is, on this hypothesis, perfectly unintelligible ; implying a sort of regularity in the lateral wanderings of the river which is absolutely in opposition to the very essence of that action. A stream quits one side of a valley, because it finds a lower level on the other; so that opposed terraces, instead of being on the same level, are necessarily at different ones. Under this hypothesis, the lines traced round the Cul de Sac at the head of Glen Roy, could not have existed at all. This must have remained a solid mass of alluvia, as it is impossible that a river, under any circumstances, could have found its way into a recess of this nature. To these remarks we may add, that, in Glen Fintec, as well as in Glen Turit, there is a point of rest where no water could have flowed on any supposition, and where, nevertheless, the lines are as distinctly marked as in any part of Glen Roy, where it is supposed to have flowed freely.
The state of the surrounding country. as already describ ed, is sufficient to prove that no river could have ever ex ssted capable of producing the requisite changes. No river could have produced the line near Loch Spey, under any imagined former condition of the country, even had one existed; and that no river ever could have flowed in this manner, is perfectly certain, or it must have been there still. Long after this, where the Roy really begins, it is a feeble stream, utterly incapable of producing the effects in question. We think it quite unnecessary to pur
sue this inquiry any further. We need'only add, that we have given it every possible advantage, by limiting these arguments to Glen Roy. Had we extended the same inves tigation to Glen Spean and Glen Gloy, it would have been only with the effect of accumulating objections of the same nature ; and, in some instances, even more insurmountable, if that be possible. We shall now, therefore, proceed to examine the hypothesis proposed by Dr. Macculloch, which attributes these lines to the drainage of ancient lakes, and which is so fully adapted to explain all the appearances, that it has set the question at rest for ever.
We admit that there are some difficulties, and of no trifling magnitude, which attend this supposition also ; but none of them are contrary, either to geological or mathematical laws, nor do they even approach to physical impossibility. Direct proof there can of course be none ; but, in cases like this, we can only reason at any time from analogies. At the same time, we may here make use of the argument by dilemma, to which we are reduced by the rejection of the former hypothesis. In reviewing the direct arguments in its favour, our readers will perceive that some of them must have been in a great manner an ticipated in examining the former hypothesis, so that our details will thence admit of being somewhat the less par ticular.
The absolute water level which exists between the cor responding lines, whether in Glen Roy or the adjoining valleys, admits of a most ready and obvious solution, by supposing that these were once occupied by lakes, nor in deed can it be explained on any other supposition. Omit ting Glen Gloy for the present, as a doubtful connection, we find that a free communication exists throughout the other valleys ; so that it is easy to imagine the water re placed in them in the same situation at some one point, where they would form one intricate lake. The boundary of the surface of that lake would be the lowest line, which we have shewn to be the only one of which the extensive continuity can really be traced. That boundary is now, of course, deficient in this case, where the bottom of the val leys has an exit beneath that line, but this will be a sub• ject for future consideration.