It has been shown, that the level of the upper line of Glen Roy is far higher than that of many of the valleys, or than much of the surrounding land which would now afford a passage to the waters of this ancient lake into the sea. No water could now therefore stand at that level, unless all these openings were obstructed, or raised to at least a higher elevation. The same holds good for the lower levels, whether in this valley or in that of the Spean. To determine the position of these barriers or obstructions is the first point necessary, and it is attended with no small difficulties ; with difficulties indeed so complicated, that we cannot pretend to surmount them in our present limited state of knowledge respecting the former changes of the earth's surface. We shall first confine this inquiry to Glen Roy, as forming the principal picture. It is difficult enough here to understand the nature of this obstruction, or the former state of the ground; but that difficulty is much in creased, when Glen Spean and Glen Gloy are taken into consideration.
It has been shown, that if water was accumulated as high as the uppermost line of Glen Roy, or so as to form the first Loch Roy, it would flow for a certain depth by the eastern extremity of the valley, or over Loch Spey, to the eastward. '1 here also, had it communicated at the lower extremity with Loch Laggan or Glen Spean, it might flow out at the eastern end of that lake, and that even it had subsided to a far lower point, namely, to one not far re moved above the level of the lowest line. But it would also flow, and that more completely than by the former route, into the Caledonian Glen, through Glen Spean, so as to find its way into the Western Sea, either by Loch Shiel, Loch Eil, or Loch Ness, or by the whole of these openings It is evident, therefore, that not only the con dition of the present apertures of these valleys must have materially differed from the former one, but that all the ad joining country must have been different from what it is now. Every one of these apertures must at any rate ha e been closed by obstructions or barriers of some kind.
To attend, in the first place, to the eastern extremity ; there is not much land required at Loch Spey, to elevate that point sufficiently to render it possible for water to stand at the highest level of Glen Roy, as far as this point is concerned. if there is any difficulty in accounting for the subsidence of this point, it is a mere trifle compared to the rest ; nor need we dwell on it, as the loss of water by this aperture, should it have occurred, could have little effect as to the phenomena in general.
The question that relates to the eastern barrier of Glen Spean is far more difficult, if we suppose that Glen and that valley formed a common lake at the highest levels. But as there are no traces of the two upper ones in this valley, there seems no necessity for adopting a supposition which only augments our difficulties. In the lowest con dition of both they unquestionably formed one lake, be cause the lowest line is continuous through both. The two upper states of the lake may be supposed to have been confined to Glen Roy. If we were to adopt the opinion, that Glen Spean and Glen Roy formed one common lake at the highest level, it is evident that the eastern barrier of the former, near the head of Loch Laggan, must have been much higher than it is now ; namely, the difference, and somewhat more, between the highest and lowest lines, or 294 feet. Should such a supposition be adopted, it is plain that some considerable change must have taken place in the state of this barrier, and from causes which we have no means of assigning. But as the same difficulty exists in examining the western outlets, and as there is no occa sion to multiply these, we think it a more safe conclusion, that these formed the great exit of the confined waters, and that no very material changes have occurred either at the east end of Loch Spey or Loch Laggan.
If now we turn our attention to the western end of Glen Roy and Glen Spean, we shall find that they unite in one common valley, which communicatesby a very wide open ing with the great Caledonian Glen. Both Glen Roy and Glen Spean also bear the continuous marks of the lowest line, which further extends far down this common wide valley, to a point at no very great distance from where it loses itself in the great glen as it opens into Loch Eil. There has therefore been a lake at the level of the lowest line, common to Glen Roy and Glen Spean; and conse quently the barrier by which it was confined, must be plac ed at a point at least beyond Teindrish, where this line ceases on both sides. Now, such are the nature and form of the ground, that it is impossible to imagine any such bar rier placed here, without its occupying even the vale, or great flat of the Lochy. This is one of the first great dif ficulties that occurs in assigning the place of this obstruc tion, and it is possible that it ought to be removed to some point more distant. This should particularly be the case, if in Glen Gloy and Glen Roy the lines were on the same level, as these also should then have formed a common lake. 13u i,respecting this point, it has been seen that there is yet some doubt.