350 Dr. Mac Culloch on the Parallel Roads of Glen Roy. 



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 inti- 

 mate 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 con- 

 tinuing 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 



