Tracks of Animals in the Forest Marble. 543 



the motion of the water drifts forward the grains of sand, &c., 

 as in a flowing tide or a river current, the ripples will advance 

 by a sort of rolling motion one over the other. These circum- 

 stances produce the compound forms of ripple-marks, sections 

 of which are to be seen in the diagrams annexed. 



The waves observable on running water are occasioned by 

 the resistance either of the sides or bottom against which it 

 impinges, and will be greater in proportion to the roughness of 

 these resisting surfaces, under similar circumstances of velocity 

 and volume. The impulse which produces the waves in run- 

 ning water is its gravity urging it down an inclined plane ; the 

 elasticity of the fluid causing it, after yielding to this impulse, 

 to rebound from the bottom or sides against which it strikes ; 

 and the rebounding wave will be higher and larger in propor- 

 tion to the force of the impulse. Shallow water, flowing with 

 a certain velocity, thus moves in a series of bounds or ripples, 

 whose direction and size are determined by the nature of the 

 sides and bottom of the stream, and which remain constant in 

 the same spot as long as these circumstances and the velocity 

 and volume of the current remains the same. 



Thus, in the diagram below, representing a stream of run- 



ning water, which, meeting with a large stone , is flung 

 upwards in a constant wave c, which is broken at the top if the 

 obstacle is considerable, and falling downwards excavates a 

 hollow at 6, immediately behind the obstacle. From thence 

 the water rebounds upwards in the form of the wave d, which 

 also usually has a tendency to break, and falling thence, con- 

 tinues its course in a series of waves, gradually diminishing in 

 height until the impulse to a vertical oscillation, originally com- 



