Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/360

 from phenomena of which we see or imagine the causes going hand in hand with the effects. These phenomena are not uncommon in mountainous countries, and must be familiar to those who are intimate with the highlands of Scotland.

The sudden and rapid rise of a torrent when fed by the streams from the neighbouring hills, is marked by lateral devastation and ruin, wide in proportion as the materials of the hills are subject to be removed and transported by the rapid flow of the water. Clay, gravel, and stones thus transported are deposited in banks which skirt the course of the torrent at the level of its highest elevation, often continuing to mark that course to future times, till a fresh flow of water rising to a higher level destroys them or substitutes higher banks in their places. A partial deluge is but a torrent on a greater scale, and we can therefore conceive this phenomenon extended in its magnitude and consequences, but producing a similar set of appearances. This cause has been supposed to have produced the lines of Glen Roy.

The banks of the Spey, of the Lyon, and those of almost every river running through a flat alluvial valley in Scotland, or (to make use of an expressive Scottish term,) a strath, exhibit the appearance of terraces, at different elevations and distances from the present course of the stream, which have an abrupt edge resembling the profile of an earthen military defence. The numerous elevations which these terraces assume in any one valley, prove that the cause by which their surfaces have been levelled has acted at various successive intervals downwards; while their variously placed lateral sections equally show that the cause of their waste at the sides has acted at many different periods laterally. If we now attend to the course of a stream through an alluvial and flat valley of this nature, we find it gradually, often imperceptibly, but sometimes suddenly changing its position in consequence of partial obstructions produced