OF THE earth's SURFACE. 171 



Wliere the obstacle is high and narrow ; where its height, for 

 instance, is at least equal to its breadth, we perceive that a 

 stagnation of the fluid takes place on the side towards which 

 the stream is flowing, and a deposition is formed of the trans- 

 ported substances, constituting a tail or prolongation, which ex- 

 fends in the direction of the stream, by a gradual descent to 

 the distance frequently of eight or ten times the height of the 

 obstacle. At the same time, an acceleration is occasioned at 

 certain places in the neighbourhood, by which the general de- 

 position, which was going on at the time, and which would 

 have been universal had no interruption occurred, is there 

 prevented. This acceleration takes place along that face of 

 the obstacle which fronts the stream, along both its sides, and 

 along those of the tail, forming by the stagnation just men- 

 tioned : the consequence is the production of a hollow or de- 

 pression in all those places, below the level of the general de- 

 position in the neighbourhood. The case is different where 

 the obstacle is of great breadth compared with its height. 

 Wlien its breadth, for example, is five or six times greater than 

 its height, the effect is often reversed, and an excavation is 

 occasioned on the side towards which the stream is flowing. 



The action of a current of water upon sand, or of wind upon 

 snow, previously deposited, produces effects similar to those 

 just pointed out. Thus, a firm body, occurring in the midst 

 of such an assemblage, mitigates or prevents the action be- 

 hind the obstacle, that is, on the side towards which the cur- 

 rent is flowing, and, at the same time, augments the corrosive 

 energy in front of it, and along the two sides. Effects are 

 thus produced similar to those in the case last mentioned ; and 

 it may be difficult in many cases to determine whether a par- 

 ticular assemblage has been produced, by a modification of the 

 first deposition, or by ^ second action upon an assemblage pre- 



Y 2 viously 



