ORIGIN AND AGE OF THE SUITS HEAT. 275 



is measured by the rate at which sediment is carried 

 down by our rivers. Consequently, in order to deter- 

 mine the present rate of sub-aerial denudation, we have 

 only to ascertain the quantity of sediment annually 

 carried down by the river systems. 



Very accurate measurements have been made of 

 the quantity of sediment carried down into the Gulf 

 of Mexico by the River Mississippi, and it is found to 

 amount to 7,474,000,000 cubic feet per annum. The 

 area drained by the river is 1,224,000 square miles. 

 Now, 7,474,000,000 cubic feet removed from 1,224,000 

 square miles of surface is equal to l-4566th of a foot 

 off the surface per annum, or 1 foot in 4566 years. 

 The specific gravity of the sediment is taken at 1*9, 

 and that of the rock at 25 ; consequently the 

 amount removed is equal to 1 foot of rock in about 

 6000 years. For many reasons there are few rivers 

 better adapted for affording us a fair average of the 

 rate of sub-aerial denudation than the Mississippi. In 

 this connection I may here quote the words of Sir 

 Charles Lyell : — " There seems," he says, " no danger of 

 our over-rating the mean rate of waste by selecting the 

 Mississippi as our example, for that river drains a 

 country equal to more than half the continent of 

 Europe, extends through 20 degrees of latitude, and 

 therefore through regions enjoying a great variety of 

 climate, and some of its tributaries descend from 

 mountains of great height. The Mississippi is also 

 more likely to afford us a fair test of ordinary 

 denudation, because, unlike the St. Lawrence and its 

 tributaries, there are no great lakes in which the 

 rluviatile sediment is thrown down and arrested on its 

 way to the sea." * 



* " Student's Manual of Geology," p. 91 (second edition). 



