Should the barrier in question have been thus distant, it must have happened that part of Glen Lochy should have been included in a common lake with Glen Roy. If also Glen Gloy is included, then that boundary must have lain to the north of the opening of this valley, and beyond the limits of Loch Lochy, which must of course have been in eluded in this general lake. But when we attempt to de termine the true point beyond Loch Lochy, we become entangled in insurmountable difficulties, nor is it possible to fix on one that will satisfy the requisite conditions.
If now we turn our attention to the southern or lower end of Loch Lochy, we find it terminating in a wide allu vial plain, communicating by large openings with the sea at Fort « illiam, and with the wide valley in which the western branch of Loch Eil lies. If it is necessary to select a place for a barrier here, and it is plain that, under this view, more than one is required, we should be led to chose the narrowest part of this opening, which lies at Fort William, between the skirts of Ben Nevis and the opposite hills of Ardgawar. The aspect of the ground, the course of the waters, and the nature and disposition of the rocks, render it difficult to select any other point nearer to the opening of Glen Spcan.
But as there is another free passage to the sea here through Loch Shiel and Loch Moidart, another barrier must he conceived to have existed in this place. This would imply one ancient lake, occupying Glen Roy, Glen Spean, Glen Gloy, the great Caledonian Glen, from some unknown northern point to Fort William, Loch Arkeig, and the western part of Loch Eil, to some undefined point towards the Western Sea. This supposition rests on the probability of Glen Gloy having been included. if that is omitted, the extent of this imaginary lake becomes pro portionably diminished.
One of the minor difficulties in this supposition, is the 'absence of water-marks or lines round those hills which include the valley of the Lochy, as these terminate even before Glen Spean joins it, and are found nowhere after wards. That absence would not in itself, however, offer an unanswerable argument against this position of the sup posed barrier ; as, in so many other places, these lines are wanting where they originally must have existed. Never theless, those who are inclined to think otherwise may, if they please, suppose that this barrier has existed some where about the place where the lines of Glen Spean now terminate. But, wherever this dam or obstruction did exist, it must, in the first instance at least, have given way sud denly, to permit the waters to quit the shore which forms the present line, however the remainder it may have been worn away by subsequent operations. That no water remained long pent up after this first breach, is rendered probable by the absence of any lines beneath this lowest one. We must now return to Glen Roy, as far as it appears to have been independent of this larger lake occupying Glen Spean.
The complete and sudden transition from the first or uppermost to the second line of Glen Roy, and the simi lar transition from that to the lowest, show that Loch Roy had subsided at two different intervals, before that third and last subsidence which emptied alike the lake of Glen Spean. Now, as we already hinted, since no marks of these two upper lines are found in Glen Spean, and as it possesses no barrier to the cast, by which water could there have been dammed at this height, we need not suppose that it participated in this lake of Glen Roy at these two early periods. We must therefore discover a new barrier
for this purpose at the west end of Glen Roy itself, on the supposition that at its two upper levels it formed a distinct lake, and that it only became united with the lake of Glen Spean at the lowest one, or after the second subsidence of its waters.
This, of course, must have existed at the very exit of Glen Roy, as the upper lines are both marked to no great distance from that point. This barrier, also, like the for mer, must have failed suddenly at two distinct and distant intervals. Neither could much time have intervened be tween these, otherwise the lines would have been less strongly marked, while intermediate ones would also have been found. We admit that by thus supposing two west ern barriers in different places, instead of one, the difficul ties are increased ; but this supposition is necessary on the foregoing view of the original state of Glen Spean.
It is proper'to ask, to what causes the failure and dis appearance of these barriers could have been owing; nor can we assign any, consistent with our knowledge of the actual revolutions of the earth in this place, but the cor roding power of the streams issuing from these lakes ; if indeed that will account for the apparent suddenness of the event, and the great decision of the intervals. What ever causes may however have led to the first evacuations of these waters, the gradual action of the rivers on the surfaces of the ground since their drainage, is perhaps quite sufficient to account for the present appearance of the valleys and the total disappearance of these ancient boundaries. It is possible also to conceive that these bar riers might have been of such a nature as to have been suddenly broken down to the different levels by the mere weight of the water above them. Earthquakes and con vulsions, as is usual on similar occasions among vulgar geologists, have also been suggested as the probable causes of these phenomena ; and thus the failures of the barriers in question hate been supposed to be connected with some imaginary convulsion which formed the Great Ca ledonian Glen. It is possible that some such event may in ancient times have separated these two parts of Scot land ; but we are incompetent to reason to any purpose, where we have neither facts nor analogies to guide us. Whatever may be thought of this, it is scarcely possible that such an event should have taken place within that pe riod, or rather those periods, which evacuated the waters of these lakes. Three such convulsions must have been produced, and these at immense intervals of time ; as is indicated by the nature of the shores for each successive lake. Of such there is no probability; and most assur. — ly not within any period during which the present ge ;era state of the surface existed. Our present rivers, alci out present alluvia, have been determined and mo ife t. what they are now, since any great changes of this na Neither, if we imagine such a convulsion limited to the last failure of the barrier, could it possibly have occurrc without entirely disturbing the condition of the surround ing country. More particularly, it must have disturbed the noted regularity of those appearances in Glen Roy which existed before it. This argument we think quite satisfactory against the opinion that attributes these events to earthquakes or convulsions, and therefore we shall dis miss this part of the subject.