56 THE WORLD'S LUMBER ROOM. 



great trench, 300 feet deep and from less than a mile to 

 eight miles wide, and the whole of this solid stone it has 

 ground into mud. On an average it brings down nearly 

 131 cubic feet of sediment every second, so that it would 

 take about nineteen seconds to fill a room seventeen feet long, 

 fifteen wide, and ten high. Much of this sediment consists 

 of the rich soil washed down from the highlands of Abys- 

 sinia, and to it Egypt owes her wonderful fertility ; for year 

 by year it is spread over the whole surface covered by the 

 inundation, renewing the land, which never need lie fallow 

 nor have any artificial aid to render it fertile, though the 

 rate of increase is only about four inches and a half in a 

 century. 



The Mississippi transports a larger load than the Nile, 

 147 cubic feet per second ; while the Ganges outdoes them 

 both, carrying as much as 557 cubic feet. 



Compared with these large rivers, the Thames seems but 

 an infant, yet in the course of a twelvemonth it manages to 

 carry down the very respectable load of 1,865,900 cubic 

 feet of solid matter, and the Danube, Po, and Rhone, 

 convey several hundred times as much. 



Since the greater part of the mineral matter carried away 

 by rivers is ground extremely fine, as we have seen, it is 

 quite evident that most of it must be conveyed into the 

 ocean, for a specimen of Rhine water, though kept perfectly 

 still, has been found to take four months to become quite 

 clear, and the sediment could not therefore by any possi- 

 bility have time to settle while being carried down by the 

 river in the few days that would elapse before it reached 

 the sea. 



