[28 AOTION OF RUNNING WATERS. 



base, and the quantity of fallen matter to be removed, must have varied. 

 At some points it may have receded much faster than at present, at clhers 

 much slower ; and it would be scarcely possible to decide whether its ave- 

 rage progress has been more or less rapid than now." LyeWs Travels in 

 North America. 



10. Effects of weight. Water acting by its own weight like 

 other bodies, evidently often contributes to such land-falls as we 

 mention, and also exerts a powerful action on the dykes and bar- 

 riers which retain it. We see the unhappy effects of inundations, 

 to which certain countries are subject from their vicinity to rivers, 

 lakes, or seas, retained by natural or artificial dykes. 



11. Jjlcfion of running waters. To the softening action and 

 weight of waters is often added a new power, from the motion 

 they acquire by running over steep descents. This force is some- 

 times prodigious. The effects are seen after storms which pass 

 over moveable substances, in the deep ravines found to have been 

 excavated. These effects are in proportion to the mass of water, 

 and the rapidity of its motion on a particular point. When a hur- 

 ricane or violent storm bursts on a mountain, the soil is often found, 

 unless it consist of living rock, removed and gullied to great depths. 

 The numerous fissures on the surface of rocks facilitate the action 

 of waters, and a considerable mass of fragments is soon detached, 

 which increase more and more the destructive power of the current. 

 Then blocks of every size are loosened, torn from the mountain 

 and transported to great distances, multiplying the effects ten or 

 even a hundred fold, in proportion to their mass and rapidity of 

 motion. Hence we have great ravines on slopes that were pre- 

 viously unbroken, and an immense accumulation of debris at the 

 foot of the mountain, and especially where the soil or the swiftness 

 of the stream abated. Torrents swollen by circumstances of this 

 kind, or by the sudden melting of snows, also produce frightful 

 ravages; they sweep everything in their way, even the living rock, 

 which they soon attack forcibly by the fragments and blocks they 

 swiftly urge along. Nothing is more terrible than this kind of 

 water-course, and to form an exact idea of the effects one must see 

 a gorge through which it has passed, sometimes rolling along rocks 

 measuring ten or fifteen cubic yards. 



12. Debacle of Lakes. Lakes which sometimes form in valleys, 

 by avalanches or falls of land, constituting a barrier which retains 

 them, are most fearful in their debacle (sudden escape of their 

 waters from breaking of their barrier), in consequence of an enor- 

 mous mass of water rushing forth in a few seconds. Scarcely does 

 a flow begin through a few rents, before the first opening rapidly 

 enlarges, and in an instant the whole dyke is carried away. An 



10. Does the weight of water contribute to its effects ? 



11. What are the effects of running waters ? 



12. What is meant by debacle ? What are the effects of debacle ? 



