100 PHILOSOPHY OF ZOOLOGY. 



When we look at a river after rain, emptying its con- 

 tents into the sea, we perceive that it has brought along with 

 it a considerable quantity of gravel and mud, which it de- 

 posits in the form of bars, sand-banks or deltas. This mud 

 has been obtained from the disintegration and wearing down 

 of the rocks through which it has passed ; and contributes 

 to fill up the ocean, by forming land on its borders. The 

 flat ground at the mouths of the Ganges, the Nile, and the 

 Rhine, have derived their origin from this source, as well 

 as the carses of Falkirk and Gowrie in Scotland. If the 

 attention is turned from the sea to inland lakes, we observe 

 the same process of upfillmg going on, with the assistance 

 of other circumstances connected with their condition. 

 Mud is constantly poured into them by the rivulets ; the 

 testaceous animals separate lime from the water for their 

 shells, which ultimately go to the formation of marl. Aqua- 

 tic plants multiply ; and, by their annual decay, form layers 

 of peat. The whole mass of foreign matter, by degrees, ac- 

 quires the altitude of the mouth of the lake, passes into the 

 state of a marsh ; and, by the wearing away of the rocks at 

 the outlet, is in part drained, becomes fit for grazing, and, 

 finally, suitable for cultivation. The rivulets now pre- 

 vented from precipitating their suspended contents in the 

 lake, carry them to some lower pool, or farther on to the 

 sea. Numerous plains, meadows and peat-bogs, indicate 

 the former operations of this process ; and in every lake at 

 present, similar changes may be observed taking place. 



This obvious tendency of the present order of things to 

 wear down eminences, and fill up hollows, has not been con- 

 fined to the period of the formation of the alluvial strata, 

 but has exerted its influence during the period of the for- 

 mation of all those rocks in which organic remains are im- 

 bedded. Thus, when the position of the beds of the transi- 

 tion rocks are examined in the great scale, they are found 



