THE MAMMOTH CAVE. 53 



It may be well in this place to refer to the 

 interesting relation subsisting between Mam- 

 moth Cave and Green River. There can be no 

 .doubt that Green River has cut out the bed or 

 channel through which it runs ; for on ascend- 

 ing its banks on either side for a distance of not 

 less than three hundred feet, a plain is reached, 

 which is not succeeded by a valley ; establishing 

 conclusively that it has worn its bed to its pres- 

 ent level by the mechanical and chemical 

 agency of water, and that the avenues of the 

 Cave were cut through with nearly equal pace, — 

 those near the surface of the earth being formed 

 first, and the others in regular order from above 

 downward ; the avenues through which Echo 

 and Roaring Rivers run being the lowest and 

 last formed. Both of these rivers are on a level 

 with Green River, with which there is, as before 

 stated, a subterraneous communication. As 

 Green River continues to deepen the valley 

 through which it passes, the avenues of the 

 Cave will continue to descend, until the springs 

 which supply Echo and Roaring Rivers cease to 

 flow, when the avenues through which they run 

 will become as dry as Marion's Avenue, which, 

 at an early period in the history of the Cave, con- 



5* 



fc 



