$0 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 
