LESSONS IN GEOLOGY. 343 



and sometimes spreading out into lakes, and in this region the 

 largest tributaries are received. From this undulating country 

 it passes into a broad alluvial plain, mostly deposited from its 

 own waters, and winds sluggishly onward, and finally, amid 

 marshes and banks of mud and sand, it empties into the sea. 

 The great rivers of the globe probably have an average slope of 

 less than 24 inches per mile. The Volga from source to sea has a 

 slope of less than 4 inches per mile, while the Missouri has a 

 slope of about 28 inches per mile. The water next the bottom 

 and sides of the river channel is retarded by friction, so that 

 water in the center and upper part of the river flows most rap- 

 idly. The fact that rivers flow more rapidly when in flood is 

 explained on the theory of less friction and increased slope from 

 high water in the upper courses. 



Each little rivulet, each spring in the whole valley, brings to 

 the river products of terranean and subterranean weathering, 

 which the river carries out to sea with materials gathered up by 

 erosion along its course. The amount of material carried by riv- 

 ers in this way varies greatly; rivers flowing through cultivated 

 regions usually carry great quantities of sediments. The Mis- 

 souri river, flowing through recent, partially consolidated strata, 

 is loaded with sediment, and so are the rivers of China that flow 

 through the loess deposits; while the St. Lawrence system, with 

 its rocky valley and lake basins, is nearly always clear and free 

 from sediment. The water of rivers doubtless has some chemical 

 effect on the materials over which it flows, but the chief work of 

 flowing streams is mechanical. The matter carried by streams 

 is partly in solution and partly in suspension, and partly coarse 

 material pushed along the bottom. Of the substances in solu- 

 tion, calcium carbonate is most abundant, then calcium sul- 

 phate, sodium chloride, magnesium carbonate, and sulphate, etc. 

 It is estimated that the quantity of calcium carbonate removed 

 from the limestone areas of the Thames amounts to 140 tons 

 per square mile annually. 



The power of a stream to transport suspended matter is said 

 to increase as the sixth power of the velocity of the current, so 



