20 The Ma\e'Up and the Wor\ings of the Earth 



It is a comparatively simple matter to determine how 

 many thousands of tons of silt and sand are carried into the 

 Gulf of Mexico by the Mississippi River every year. From 

 such facts it has been calculated that the entire Mississippi 

 River basin would lose a quantity of soil equal to a layer 

 one foot deep in the course of 6000 years. As an actual fact 

 soil is being removed much more rapidly in some regions, 



Fig. I. Massive Erosion in Mongolia 



The bad lands at Urtyn Obo show the effects produced during the ages by the 

 action of rain and flowing water upon the sands and rocks. From photograph by 

 the Gobi Desert Expedition of the American Museum of Natural History. 



but much less rapidly in others. The Ganges River is wash- 

 ing parts of India away at the rate of an average one-foot 

 layer in about 850 years. Soil is being thus steadily removed 

 throughout the whole world. 



By accumulating in detail numberless facts of observa- 

 tion and measurement and experimentation, scientists have 

 attempted to answer such questions with respect to many 

 kinds of soil or sand and with respect to the different sub- 

 stances that water dissolves out of the soil. With the answers 



