650 DYNAMICAL GEOLOGY. 



posits, but always sparingly ; for they are mostly destroyed by wear 

 or by decay. They rarely if ever accumulate in beds fitted for mak- 

 ing coal, being widely scattered by the currents. Fresh-water and 

 land shells are occasionally found in the beds. Remains of other 

 animals are also distributed by the waters, and buried, though seldom 

 escaping destruction, unless carried into a quiet portion of the flood- 

 plain. Thus the fossils of river-deposits may have come from a region 

 of very wide range. 



In the case of the bursting of lakes, the fishes of the waters and 

 the material of the bottom, including its shells, are sometimes trans- 

 ferred to a level below, far distant from the source. Lyell says that 

 bogs, on bursting, have sent forth great volumes of black mud, which 

 has flowed slowly along, making a deposit sometimes fifteen feet thick, 

 and overwhelming cottages and forests. 



As the range of height within which river-waters can work has 

 narrow limits, the thickness of the alluvial formations made by a 

 stream, in any given condition of it, is necessarily small. Even the 

 whole of the river-flat, above the level of its bottom, may not have 

 been deposited by the river in its existing state ; for the channel and 

 flood-plain may be excavated in the alluvium or river-border forma- 

 tion of an earlier period, so that its upper surface alone may be of recent 

 origin (p. 560). If, however, the land were undergoing a very slow 

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

 of detritus would take place, that would raise both its bed and flood- 

 plain ; and the thickness might thus go on increasing so long as the 

 subsidence continued. Moreover, when rivers flow rather sluggishly 

 through plains, they tend to raise the bottom by the deposit of sedi- 

 ment ; and, consequently, the dikes that may be built to prevent the 

 spread of the waters over the flat country during floods have to be 

 correspondingly raised, to prevent catastrophe. 



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 fine, earthy alluvium, which is formed by a slow deposition of detritus by annual 

 floods, consists of thin, even layers. A vibration or wave-movement, in the waters of a 

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

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

 and upper portions, the progress of the wave. But the thin layers of alluvium usually 

 mark the depositions of successive floods. 



The pebbles or stones, forming beds in alluvium, ai - e 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 wa} r be made pebbly, while the 

 Dpposite is muddy, or has a sand-bank forming, from the earth carried across. 



