180 ON THE REVOLUTIONS 



conceive that they have in fact been produced by the action of 

 eddies of inferior force to tlie main stream, but acting in com- 

 pany with it in different, and sometimes opposite directions. 

 These various corrosions going on together, have each produced 

 its pecuhar effect ; and most of them being concave, their 

 meeting has given rise to the set of waving angular ridges 

 which constitute the most unequivocal feature of a water-worn 

 rock. These angular forms differ completely from those which 

 occur in the broken surface of a rock. These last are acute, 

 rectilinear and abrupt ; while those others are continuous, 

 flowing, and having their angles very obtuse ; so obtuse in 

 some cases, as not to be visible, unless the light strikes upon 

 the rock in a peculiar direction *. 



In a small but rapid brook near my house in the country, 

 these forms occur on the surface of a smooth bed of sandstone 

 over which it flows. I observed lately, when the brook was 

 low and clear,, that, to a certain distance below each of these 

 obtuse-angled ridges, the rock was covered with green moss, 

 while above the angle it was bare. The mode in which these 

 forms are produced and maintained, seems thus to be pointed 

 out : the main stream being possessed of just power enough 

 to keep the rock clear of moss, and the eddy being too weak 

 for this purpose. In a flood, I presume that the eddy acquires 

 such power, that the whole rock is cleared* 



In 



* I have endeavoured, as yet without success, to make or procure an adequate 

 representation of these forms ; but I do not despair of overcoming this difficulty, 

 and of being able, on some future occasion, to lay something of the kind before 

 this Society that may not be unworthy of their notice. In this undertaking, I 

 hope for the assistance of a very masterly artist, who is interested in the investi- 

 Tatioii, and who has already rendered me great services in this and otlier pup- 

 siiits. 



