WATER. 645 



and flood-plain may be excavated in the alluvium of an earlier 

 period, so that the upper surface alone may be of recent origin 

 (p. 550). If, however, the land were undergoing a very slow subsi- 

 dence, which should diminish the pitch of the stream, a deposition 

 of detritus would take place which would raise both its bed and 

 flood-plain, and the thickness might thus go on increasing as long 

 as the subsidence continued. 



The deposition of detritus which takes place along the course of a river 

 usually raises the borders of the channel above the general level of the flood- 

 plain. Along the Lower Mississippi, the pitch of the plain away from the 

 river amounts, on an average, to seven feet for the first mile. (Humphreys 

 & Abbot.) 



The earthy alluvium which is formed, by a slow deposition of detritus con- 

 sists of very thin even layers. A vibration or wave-movement in any waters 

 in which a sediment is falling tends to arrange that sediment in layers, each 

 layer corresponding to a wave, and showing by a difference of texture in its 

 under and upper portions the progress of the wave. In the case of accumula- 

 tions from a rapid deposition or pressing forward of material, the lamination is 

 often wanting. 



The pebbles or stones forming beds in the alluvium are brought in by the 

 upper waters and lateral tributaries during floods. The course of a tributary 

 across the river-plain is often marked by a wide bed of stones. The sweep of a 

 freshet over the earthy flood-plain may carry away the finer earth and leave a 

 surface of pebbles. The bank of a river struck by a strong current may in a 

 similar way be made pebbly, while the opposite is muddy or has a sand-bank 

 forming from the earth carried across. 



Still other irregularities result from changes in the river-channel. The trans- 

 fer of material from one side of a stream to the other ends often in making a 

 long bend, and finally in cutting off the bend and turning it into an island, and 

 ultimately into a part of the mainland by the filling up of the old channel. 



The islands in the large rivers are also very unstable. In the Mississippi, as 

 Humphreys & Abbot observe, they often begin in the lodging of drift-wood on 

 a sand-bar ; this causes the accumulation of detritus ; a growth of willow suc- 

 ceeds ; the height of the alluvium still increases, until finally the island reaches 

 the level of high water, or rises even above it, and becomes covered with a 

 groAvth of cotton-wood, willow, etc. By a similar process, the island may be 

 united to the mainland ; or, " by a slight change of direction of the current, the 

 undei'lying sand-bar is washed away, the new-made land caves into the river, 

 and the island disappears." 



2. Delta formations. — The larger part of the detritus of a river is car- 

 ried to the ocean (or lake) into which it empties, and it goes to form 

 about the mouth of the stream more or less extensive flats. Such 

 flats, when large and intersected by a network of water-channels, 

 are called deltas; they reach a large size only where the tides are 

 quite small or are altogether wanting. They are formed from th§ 

 conjoined action of the river and the ocean, and are sometimes 



