Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/393

 of the water. But if we examine any one of these furrows we find that a portion of it, one half perhaps, and that of course the most ancient as the most superficial, bears the unwasted mark of the lines while the interior part, subject to a more modern destruction, exhibits no such impression. It follows therefore that the lines are posterior in time to one portion of the period through which the water has acted, and prior to the other. But if we now conceive the water of the lake to have stood at the height of the upper line, it is evident that the furrow itself could not have been formed at that level and so far below it as to admit of a regular deposit of a shore, such as I have supposed the lines to be, on its sides; as well as the repetition of two similar deposits occupying precisely the same portion of the furrow, at one or two lower levels. Since the descent of the torrent must have ceased at its contact with the water, that part of the furrow which, from its now bearing the mark of lines, is evidently of higher antiquity than the lake, could not have been then formed; nor on the two subsequent sinkings of the water can we conceive it to have been prolonged through intervals of time so precisely equal, and with actions so precisely similar, as to produce the appearances now visible. It is a more probable supposition that the water which stood at the levels of these three lines entered within the margin of the furrow so as to deposit its shore on all that part which was then excavated; while the bottom which now shows no mark, has been formed by the action of the same torrents since the final subsidence of the waters. Water courses therefore have existed in times more ancient than the lake, extending from the summits of the hills at least below the present lowest line. But as these could not have existed but in the absence of the lake, it follows that the lake itself was of a posterior formation, and that the barriers which contained it were