Rivers. 7 



a powerful mechanical effect on the surface of the 

 earth, carrying much sediment into water-courses, 

 which unite to form brooks, rivulets, and finally, if the 

 country be large, great rivers. Soft surface soil is thus 

 easily carried away even in low countries, and in hilly 

 and mountainous regions sands, coarse rounded gravels, 

 and boulders, won from the adjoining rocks, are hurried 

 onward ; and thus it happens, that great valleys and 

 ravines have often been formed in all parts of the world 

 by running water, and by the long-continued attrition 

 of stones driven onward by torrents over rocky surfaces. 

 As the accumulated waters of rivers reach low lands, 

 their power of transporting coarse sediment decreases, 

 and finally, in great rivers, like the Rhine, the Nile, the 

 Amazons, the Mississippi, and the mighty rivers of 

 China, India, and Northern Asia, all but the finest sedi- 

 ment is deposited long before they reach the sea. 



On a smaller scale the same kind of phenomena 

 are obvious in such English rivers as the Thames, the 

 Severn, the Ouse that flows through York, and the 

 Clyde and the Tay, in Scotland. Every river, in fact, 

 carries sediment and impurities of various kinds in sus- 

 pension or held in solution, and this matter, having 

 been derived from the waste of the lands through which 

 rivers flow, is carried to lower levels. Thus it happens 

 that when rivers empty themselves into lakes or, what 

 is far more frequently the case, into the sea the sedi- 

 ments which they hold in suspension are deposited at 

 the bottom, and, constantly increasing, they gradually 

 form accumulations of more or less thickness, gene- 

 rally arranged in beds, or, as geologists usually term 

 them, in strata. Suppose a river flowing into the sea. 

 It carries sediment in suspension, and a layer will fall 

 over a part of the sea-bottom, the coarser and heavier 



