THE CENTRAL BASIN OP TENNESSEE. 105 



much greater operations have been carried out in much shorter 

 periods of time and with no more effective agents. Numerous 

 instances might be given, but one will be sufficient. According to 

 Powell and Dutton, over the whole Plateau i-egion, an area of not less 

 than 200,( 00 square miles, an average of 6,000 to 8,000 feet, and an 

 extreme of 12,000 feet has been removed by erosioi.-. since the middle 

 Tertiary. All this work has been performed in a little less than half 

 the time consumed in digging out the area covered by the plains of 

 the Highland Rim and the Central Basin of Tennessee. 



