THE CENTRAL BASIN OF TENNESSEE. 



97 



IV. Table showing the excessive and heavy rainfalls at Nashville, 

 from 1871 to 1887. 



The nature of the underlying rock has a greater or less effect upon 

 the rate of erosion. In the case of hard material the rate must 

 necessarily be slow, but where the underlying rocks are soft or the 

 material consists of clay or gravel, the operation of eroding a valley 

 or basin will be comparatively rapid, and erosion will go on until an 

 equilibrium has been established between the rate of weathering and 



