WATEK AS A MECHANICAL AGENT. 167 



Russia, on others in southern France, on the streams intersecting the low 

 land of the Atlantic border of the United States (Kerr), and on those of 

 southern Long Island (E. Lewis). 



It is shown that in streams the difference between the surface and bottom 

 velocity accounts for this erosion of the right bank, with deposition at the 

 left, thereby making the right steeper and placing the deepest part of the 

 stream near it. The extremely slow transverse motion will be combined 

 with that down stream, so that the actual motion will make a very 

 small angle with the direction of the channel. (A. C. Baines, Am. Jour. 

 Sci., xxviii. 1884.) 



(2) Kinds and methods of work. — The kinds of work done by the 

 mechanical action of waters, whether in rivers, lakes, or oceans, are in a 

 comprehensive way (1) Denudation ; (2) Transportation ; (3) Deposition 

 of the transported material, making usually stratified deposits. 



Denudation. 



Denudation is going on wherever any rock materials or rocks are within 

 reach of moving waters. It is called erosion or exca'oation, when the work is 

 the making of valleys, and degradation, when it is the wearing down of hills 

 or mountains. But the term denudation covers both processes. Another 

 style of work under it is that of planation, or the making of flat surfaces by 

 • the shearing action of spreading waters, and by deposition up to the surface, 

 or to a common level. The worn material derived from the wear of rocks is 

 called detritus, because made by wear ; and also after deposition, sediment, 

 because deposited usually from waters. Sedimentary rocks derive thence 

 their name. Silt, the finest of mud, occurring in the bottom of estuaries and 

 elsewhere, and ooze, soft, sticky mud, are extreme results of the grinding 

 process. The term deposit is a general one for an accumulation made by 

 any natural method. 



Denudation depends for its effects on the varieties and conditions of the 

 rocks subjected to it not less than on the powers of the agent, water. It is 

 facilitated not only (1) by softness or fragility of terranes, but also by 

 (2) their subdivision into thin layers ; (3) a loose junction of layers ; 

 (4) alternation of yielding layers with firmer layers ; (5) vertical joints or 

 fractures, and especially multitudes of surface cracks or rifts. (6) Boldness 

 in position is also favorable ; for high bluff fronts feel the force of blows of 

 water proportionally to their verticality, and also have gravity to aid in 

 removing loosened material, and to produce rendings where water descends 

 in vertical crevices. (7) Moreover, angular concavities or cavernous open- 

 ings and projecting points in walls give the waters great advantage. (8) A 

 horizontal position in the bedding of cliffs or walls is especially favorable, 

 because a little removal below undermines, and may cause great downfalls ; 

 and, besides, walls and cliffs are thus kept vertical, for the long continuation 

 of the work. (9) Above all, denudation is facilitated by the weakened con- 



