332 GENERAL SCIENCE 



The large quantity of earth moved by the process of 

 erosion is not destroyed, but is deposited in various 

 places for a time. The dust moved by the wind may fall 

 anywhere. The fertile valley of a river is made of the 

 soil carried from the hills and mountains. The slow 

 moving water of a river is unable to carry with it the 

 quantity of soil brought to it by its various tributaries. 

 This solid matter is spread over the flood plain during 

 each flood, and thus some of the richest farming land 

 is made. Every river valley serves as an example of 

 this rich soil deposited by rivers. 



The very fine particles of solid matter are carried into 

 lakes and oceans by rivers, and at the river's mouth they 

 settle to the bottom or are carried far out by strong 

 currents which pass the river's mouth. Deltas, such as 

 those of the Mississippi and the Nile, were formed by the 

 deposits of solid matter which the river carried to its 

 mouth. 



It can easily be seen that the hills and mountains are 

 slowly but surely being worn away and carried into low 

 lands and finally into the oceans. Some geologists say 

 that the whole Mississippi basin is being carried to the 

 Gulf of Mexico at the rate of one foot in 5000 years. 



228. How to Protect Soil against Erosion. We must 

 not think of erosion as entirely harmful. The disinte- 

 grated rock of the soil contains some elements and com- 

 pounds which are of no value to plants as food. Very 

 slow and gradual erosion removes much of this undesir- 

 able waste material so that the roots of plants can get 

 to new rock and find food. 



When erosion goes on very rapidly much of the soil 

 which contains plant food is carried away; this form of 

 erosion is undesirable and should be prevented. The 



