92 GEOLOGY. 



and it is not necessary to repeat it here. The pebbles, 

 and sand, and mud moved by water were all once in solid 

 rock, and how the rocks are thus broken up will be seen 

 hereafter. The deposit of these materials is regulated 

 by other varying circumstances as well as those which I 

 have mentioned, as I will now proceed to illustrate. 



174. Deposits in Rivers and on their Borders. A riv- 

 er descending some slope of considerable pitch always 

 brings along with it much solid matter, and if it after- 

 ward move sluggishly through a plain, this matter will 

 be deposited quite evenly over its bottom. If the river be 

 narrowed much at any point the water will move so rap- 

 idly there that it will carry the material suspended in it 

 on beyond, and then, if it spread out suddenly over a 

 large surface, there will be much matter deposited, ex- 

 cept in the direct line which the main body of the water 

 takes. On either side of this channel that is, on the 

 flats will the most of the deposit be made. In some 

 cases the plain on either side of a river is so high that 

 the water flows over it only when it is temporarily raised 

 by a freshet. This is termed the flood-plain. In this 

 case there is some addition to the plain every time that 

 a freshet occurs, until at length the land is raised so high 

 that the flood does not overflow it. This process, year 

 after year, tends to make that part of the plain immedi- 

 ately bordering on the river higher than the rest of the 

 plain, because, the farther the water is from the river, the 

 less has it suspended in it from the settling of the sedi- 

 ment which is continually going on. And as some of 

 the matter is deposited on the bottom of the river itself, 

 the river is gradually raised, and may be, after a while, 

 on a level which is above the adjoining land, except that 

 which forms the banks. These results are shown by 



Figs. 33 and 34. Let 



- 33 ' ke a flat 

 valley, and b a stream 

 Fig. 33. of water which runs 



