THE TOOTH OF TIME. 343 



prairie region of Illinois. The underlying strata are nearly 

 horizontal. Why now do they terminate so abruptly at 

 the mountain wall which we scale to reach Bon Air ? The 

 cut margins of these mountain ribs lie exposed and pro- 

 truding from base to summit of the laborious ascent. Did 

 Nature form them originally thus ? We are forced to con- 

 clude that these mountain sheets, like those under the 

 " highland rim," once extended westward over the basin 

 of Tennessee, and have been scooped out by some tremen- 

 dous agency appointed by Nature to furnish materials for 

 the states — then future — of Georgia, Alabama, and Missis- 

 sippi. The Cumberland Table-land and the abrupt knobs 

 about Chattanooga are not upheavals, but lines of relief. 

 It is the valleys that have been made, and not the moun- 

 tains. The mountains are landmarks — " benchmarks" the 

 engineer might say — showing the former level of the en- 

 tire region. 



If we travel westward or northward from Nashville we 

 find the basin walled in by the " highland rim," though it 

 is only on the east that the pile of strata rises so high as 

 to bring us within the limits of the Coal-measures. Here, 

 then, is one of the most stupendous examples of geological 

 denudation. What the precise nature of the agency by 

 which this work was done we can only conjecture. Equal- 

 ly uncertain is the precise date of the work. We can only 

 say that it was performed between the close of Paleozoic 

 Time and the present, but as to the reality and almost in- 

 calculable vastness of the work Ave have no room to doubt. 

 Neither can we fail to see that such enormous excavations 

 must have been in progress in all ages, to furnish the requi- 

 site amount of materials for formations of continental ex- 

 tent, and attaining a thickness of hundreds or thousands of 

 teet. 



I have already alluded to the monuments of destructive 



