Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/395

 boundary into the Spey, and therefore that their course westward, to which the removal of this barrier must be attributed, could not then have taken place. Independently of this consideration I showed that the difficulties of accounting for the failure of so many barriers were increased in an unnecessary degree. It is therefore perhaps a more reasonable supposition, that the barrier which dammed the lake of Glen Roy to the cast, existed beyond the point at which the waters of Loch Laggan, if now elevated above their eastern boundary, would fall into the Spey; a point situated near Dalchully. Of the nature of this boundary, as well as of the causes by which it has been removed, it would be fruitless to speculate further. It is more important to point out the magnitude of the change itself.

The depression of the Spey at this point below the uppermost line of Glen Roy, may be estimated without material error at 400 feet or more, and the valley here puts on the form of a flat strath bounded on each hand by high rocky mountains; varying from a mile to half a mile in breadth. The necessary altitude of the obstruction may hence be readily computed, and it is equally obvious that however much during the course of the Spey to the sea the breadth of this imaginary barrier may vary, according to the position in which we place it, its altitude must be constantly increasing during the descent of a river so rapid. Variations in its supposed position will produce correspondent effects on the spaces which must have been inundated during this state of things. Thus its removal a few miles lower would cause the supposed lake to fill the valley of the Truim to a considerable height above Dalwhinnie. But it is unnecessary to pursue these consequences further in this direction. It is proper however, although we cannot assign the real causes which may have produced the breaking down of this barrier, to show that independently of the general reasons assigned