IX.] THE WORK OF RAIN AND RIVERS. 147 



artificially carried over low land, in order to cover it with a 

 fine silt, called warp ; this is done in the estuary of the 

 Humber. In cases where an actual delta is not formed, a 

 bar, or shoal, may be thrown across the mouth of the river, 

 and thus interfere with navigation. A river exposed to full 

 tidal action, like the Thames, has Httle chance to form a 

 delta ; and, although alluvial deposits are to be found on its 

 banks, and shoals in parts of its estuary (Fig. 47), there is 

 sufficient scouring out of the mouth to keep its channel open. 



But, although the Thames forms no delta, the quantity of 

 detritus which it carries down from the surface of its basin 

 and discharges into the sea is far from being insignificant. 

 The proportion of solid matter suspended in water varies 

 considerably in different rivers ; and, in the same river, at 

 different seasons. Thus, Bischof, in examining the Rhine, 

 found that, when the river was turbid, it contained ^gVs' ^f 

 its weight of solid matter in suspension ; but, at another 

 season, when the water was clear and blue, it contained only 

 srlirff'^h part. The Ganges, which has formed such an 

 enormous delta, is said to hold, on a yearly average, as much 

 as 5^th by weight of suspended detritus. No river has 

 been more carefully examined than the Mississippi, and it 

 has been determined that the average proportion of sediment 

 in this great stream is x^jtj- by weight, or a-gVo by volume ; 

 so that the weight of mud actually carried to sea, in the 

 course of a year, reaches the enormous amount of 

 812,500,000,000 pounds. 



With regard to the Thames, it has been estimated that it 

 discharges annually 1,865,903 cubic feet of sediment 

 (Geikie). Add to this the quantity of mineral matter 

 washed away in solution, to which reference was made in 

 the last chapter, and it will be found that the total quantity 

 of solid matter carried to sea by the Thames is really 



L 2 



