76 THE MAMMOTH CAVE. 



limestone. The floor is covered with yellow 

 sand. 



It requires a rise of only five feet of water in 

 Echo River to overflow Great Walk ; and that 

 depth is sufficient to allow boats to float between 

 the lake and the river. There are times, we 

 are informed, when Great Walk is filled with 

 water from the floor to the ceiling. Extraor- 

 dinary as the statement may appear, it is not 

 an uncommon occurrence for the water to rise 

 to the height of sixty feet in Lake Lethe, at 

 which times the iron railing on the terrace 

 above the Dead Sea is entirely submerged. 

 This great rise of water is produced by freshets 

 in Green River. 



