Of Loch Treig, it is unnecessary to take any particular notice, as it is connected in the same train of reasoning as Glen Spean, of which it forms a part. But Glen Gloy still requires a few words, on the supposition that its only line is really elevated above the highest one in Glen Roy, of which we formerly expressed doubts, though shown to be so by our own observations. On this view it would re quire a separate barrier of its own, somewhere toward its lower extremity, as it must have formed an independent lake. One failure alone is also here requisite to answer the conditions, but we need not dwell longer on this sub ject. In concluding, we can only say, that though this part of the history of the ancient state of these valleys is thus beset with difficulties, ss e are compelled to admit the general principles, and must be for the present content to think that the foregoing views are just, however we may have yet failed in explaining all the details connected with these long past events.
The difficulties which we have thus stated relating to the nature of the barriers by which these lakes must have been retained, have led some persons to imagine a diff:r ent mode by which the waters might have been kept at the requisite height, and by which they might afterwards have been drained. This hypothesis supposes, that the forms of the valleys have been always the same as they arc at present, and that the imaginary dams or barriers were nothing else than the waters of the ocean.
If this be adntitted, the lake itself must have been a portion of the sea; or Loch Roy and Loch Spean must have been sea lochs or friths, as well as the lake of Glen Gloy. The lines found in these valleys are therefore sea shores, or the action of the sea at different levels must have produced all the appearances which we have so mi nutely reviewed. It will not require much labour to exa mine into the truth of this opinion, as many of the argu ments already used in refutation of some of the former hypotheses are equally applicable to this one.
We do not, in the first place, believe that the level of of the sea has ever undergone such changes as this hypo thesis would require. Even were we to admit, which we do not, that the elevation of the land above the sea has arisen from the subsidence of the latter, and not from the rising of the former, the present class of facts could not pos'sibly be within the limits of those actions or that pe riod of time; since they belong to that far more recent one which succeeded to these events, of whatever nature they were, and during which the present surface of the land acquired and maintained the forms it now displays.
Assuredly the great revolutions that caused the present disposition of the sea and land, were long prior to the time in which Glen Roy and the neighbouring valleys ac quired that permanence of form which is indicated by their phenomena of various kinds.
\Ve as little admit that the level of the sea has under gone any appreciable changes since the time at which the present disposition of the land was determined. That the relative level of the sea and land has changed, and is occasionally changing, we do not deny. But, in many cases, as on the coasts of Italy, this has evidently arisen from the vacillations in the state of the land and its eleva tion, not in that of the sea. Slight changes also occur in certain seas or channels, from the gradual accumulation of materials on the bottom, but there are partial as well as trilling geologists, who, while they talk with ease about alterations of the level of the sea, as if it was the sim plest of operations, forget that this cannot happen in any one place without affecting the whole ocean; and they forget also, that such changes necessarily imply the de struction or generation of whole oceans, and these often within short periods of time. That the sea should, in this place, for example, have stood on a level with the up per line of Glen Roy, it is requisite that the whole ocean should also at that period have stood 1260 feet, or there about, higher than it does now ; a state of things which must have belonged to a far different world from that which we are acquainted with.
But we know that, even in spite of such arguments as this, there are persons who still chose to iinagine such revolutions in the elevation of the sea as will admit this cause, and inquire with what success it can be applied to the explanation of these appearances. If Glen Roy was thus open to the sea, and if its lines are to be considered as ancient sea shores, the whole ocean must have under gone three successive depressions of its level at three distant intervals; since much time is required to produce Such shores as these lines must have been. At these pe riods also, it must have stood high in the great Caledo nian Glen, as well as in a vast number of the present sea lochs and valleys of Scotland, which are exactly similar to Glen Roy in their nature. Yet in no other place can the same kind of effects be discovered, nor any other phe nomena of a similar nature, which could justify such a supposition.