80 THE MAMMOTH CAVE. 



arched form, and is composed of smooth, solid 

 rock, — more closely resembling a work of art 

 than of Nature. 



From what has been said of the narrow open- 

 ing at the starting-point on the river, it may be 

 inferred that a slight increase of water would 

 render ingress impossible. There is a means 

 of escape, however, should any one be caught 

 beyond, by a small side avenue, called Purga- 

 tory, which commences at the end of Great 

 Walk, and terminates in the avenue of Echo 

 River, about a quarter of a mile from the land- 

 ing in Silliman's Avenue. A rise of eighteen 

 feet of water, however, fills the avenue of Pur- 

 gatory, and cuts off all communication with the 

 outer world. 



When there is no rise in Green River for 

 several weeks, the water in Echo River becomes 

 remarkably transparent, so much so, in fact, 

 that rocks can be seen ten or twenty feet below 

 the surface, and the additional novelty is given 

 to the voyage of the sensation that the boat is 

 gliding through the air. The connection be- 

 tween Echo and Green Rivers is doubtless near 

 the commencement of Silliman's Avenue. When 

 Green River is rising. Echo River runs in the 

 direction of Great Walk ; when it is falling, the 



