Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/379



Besides these lines, terraces of similar alluvial matters, with surfaces of considerable dimensions, and more or less level within themselves, are found in the lower parts of the valley, (vertically considered). These accompany the entrances of the different rivers into it, whether principal or lateral, and, being proportioned to the magnitude and power of the streams, they are most considerable at the entrances of the Roy and the water of Glen Turit. At the upper end of Glen Roy the most remarkable of these terraces coincide with the lowest line, with which they present a level continuously prolonged, as may be seen in the accompanying section and drawings. In these places, they are still subject to the action of the river, from which cause they are gradually wasting away and contracting their superficial dimensions; while the lateral wanderings of the river, acting now on the alluvial plain, multiplies their number, and produces a great series of inferior ones at different levels, which skirt its course for a considerable way down the stream.

It is necessary in examining the correspondence of these phenomena with the theory of a lake, to separate these latter appearances from the former. They are plainly of posterior date, and the result of an action now daily going on, derived from the powers already noticed which rivers possess of deepening their way through alluvial plains. It is only the first class of appearances that we have here to consider, for the purpose of examining how far they agree with those which are found connected with lakes at the present day.

If we examine a lake inclosed by hills of a considerable declivity, which, being formed of solid rock, are at the same time covered with alluvial matter to a greater or less depth, we find that it is