Three distinct torrents would be required to produce the three lines of Glen Roy alone, and these must be con ceived as flowing from the east toward the west, as the difficulties that attend the contrary supposition are even greater than those involved in this one. The causes of these debacles are conceived to arise in elevations of the land during former periods of the globe. To simplify this as much as possible, let it be supposed that there was but one line, and that in Glen Roy only. The imaginary wave, or mass of water, must therefore have flowed from the highest land, or the source of the Spey, in which case it ought to have flowed to the cast as well as the west. But here there are no traces of any thing analogous to the lines in question. Nor could such a torrent have pro duced even the first line in Glen Roy. A mass of water requisite to fill the whole valley for its course of near twenty miles, must have stood at a considerable height at the summit level, and could not consequently have left its traces so very near to that as we find the present line near Loch Spey to be. The very hypothesis supposes these lines to have been formed at the margin of the surface of a fluid in motion, nor could they have been produced un der water.
If, under its simple form, this hypothesis is thus un tenable, the difficulties are incalculably increased when it becomes necessary to adopt a series of such causes, or a succession of three similar waves at given distances of time, producing similar effects under such an inequality of circumstances. Although a more general and distant origin for the supposed diluvian wave were assumed, the same sort of difficulties occur ; since the obstruction to its course, formed by the elevated ground of Loch Spey, would equally interfere with the wished-for effects. If the following further arguments are not absolutely neces sary to the subversion of this ill-grounded hypothesis, they are interesting in a geological point of view.
As the lines, in many places, enter into the furrows which torrents have made in the hills, it is plain that many changes of the surface had been produced before these singular marks were impressed on them, or the torrents at least had flowed in the same channels they do now, and that during a long lapse of time. Now, the formation of the beds of these torrents is thus shown to be prior to the lines; whereas, had these latter been caused by that gene ral subversion of the crust of the globe to which the de bacles are attributed, that could not have been the cause, nor could they have preserved their forms during distur bances of such enormous extent.
It is a supposition very improbable, that three currents should have been propelled at different periods, and con sisting of different masses of water, with forces so equal as they must have been to have produced effects so simi lar; neither does this hypothesis account for the supernu merary lines, which, on such a view, could not possibly have taken place. The state of the alluvia in different
places, where these lines occur, offers another objection. In the upper or higher parts these are formed of the sharp untransported materials of the hills, while, at lower points, they consist of rounded matters, sand, and gravel. Now, the materials transported by a deluge must have been every where similar, and every where rounded alike. Nor is there any reason why such deposits should not have been formed on the rocky and hard ground as on the soft surfaces, since the inclinations of the hills arc so often similar.
It is, further, a striking objection to this hypothesis, that the thickness of these lines, or alluvial deposits, are not affected by the angular turns of the valley, or its devia This hypothesis presumes that the lines of Glen Roy and the neighbouring valleys are the remains of water terraces, similar to those common in all alluvial straths. Such terraces are the deserted banks of the streams when they flowed at higher levels, and in different places; and their slopes and forrns, resembling those ot military works, are the produce of the actions of the rivers on them dur ing their changes of place. At a distance from the sides of the hills their surfaces are flat, because they have form ed part of the alluvial plain ; while near these, their forms become combined with the slope of the ground.
As now the opposite lines of Glen Roy correspond at three several stages, it is plain that the action of the water, to produce these effects on this system, must have consisted in cutting its way through an alluvial plain from the highest to the lowest of these stages, and so at length to the present bottom of this valley. Similar effects must have taken place in all the others that are connected with it. No set of alluvia less than this, occupying the sides of the hills on the entrances of lateral torrents, could have answered the necessary conditions ; as there is no other case but that of a valley absolutely full to the highest level of any line that could have permitted the supposed river to have acted on both sides of it at such distances.
Now, if this terre plein, thus indispensable, be admitted, it is impossible that the lines should have acquired such an equality in breadth and appearance throughout their courses. The variety of ground which they occupy, and the unequal action which the water must have exerted on a set of terraces, the sides of which held an unequally angular direction towards its current, render such a sup position absolutely untenable. Neither could these lines have preserved this uniformity of breadth in all the sinu osities which they enter, as the very forms of these must have protected the original alluvia from the action of the river, and left, in these places, wide terraces.