Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/378

 remains to be added on the subject, but I shall proceed to state the very few remaining arguments on which it must rest, and oppose to them the difficulties, and the conditions now long passed away, under which such an order of things must have existed.

The absolute water level which is found to exist between the corresponding lines both in Glen Roy and in those vallies which communicate with it, admits of a ready solution, on the supposition that a lake once occupied this set of vallies; nor can it be explained on any other. As a free communication, in one direction at least, still exists among them, it would even now be easy to imagine the water replaced in the same situation: the difficulty of confining it will be a subject for future consideration. If however a lake be considered the cause, it is plain that the lines in question were once the shores of this lake; and it equally follows that it had existed at three different elevations, and that the relative depths of these three accumulations of water may be measured by the relative vertical distances of these three lines from the bottom of the valley. Thus the nature of the retaining obstacles becomes more complicated, and adds materially to the difficulties that will hereafter be examined in detail.

It is necessary in the first place to recall to the reader's mind the few facts which most directly bear on this view of the case.

We have seen that the hills which bound Glen Roy have their sides covered with a coat of alluvial matter, which in some places consists of sharp substances, appearing to be merely the ruins of the summits above; while in others it is formed of rounded ones, deposited in such a manner, as to indicate a transportation from places more distant; in these the lines are formed. The same circumstances occur in the other vallies which communicate with the principal one.