THE LAKE-BASINS OF IARCONNAUGHT. 117 



a torrent into a flattish part of the valley. As we have 

 not seen the work of one of these mountain torrents 

 described, we may attempt it. Mountain torrents 

 are very erratic in their movements : for years they 

 may flow in one course, but at any time the old 

 course may be deserted and a new one excavated. A 

 newly-excavated torrent-course down a hill-side, has all 

 the appearance of a more or less deep, artificial trench, 

 from which the materials excavated had been thrown 

 in irregular banks or heaps on both sides. At first 

 the stream takes possession of some sort of break 

 in the surface soil, whether a crack due to contrac- 

 tion, a path worn by sheep or cattle, a rut exca- 

 vated by lightning 1 or the falling of a waterspout, 

 or a trench cut by man. This it deepens and widens 

 at the bottom, till eventually, during sudden floods, 

 the stream forces up large masses of the surface, and 

 scatters the debris along the banks of the channel ; 

 or the water may carry some of the lumps along 

 with it, till caught in one of the narrows of the exca- 

 vation, when they form a dam. While the dam exists, 

 the rush of water from above will force all transported 

 materials, such as stones, turf, and the like, up on 

 to the banks on both sides. Such dams rarely last 

 long ; but as they are continually forming and burst- 



1 The ruts due to the lightning observed on the hills in larcon- 

 naught had an appearance very like a regular furrow opened by a 

 plough. 



