EDITORIAL. 293 
to the river course, then descended from bed to bed to the bottom 
of the falls. The porous, soft nature of the rock favored the 
rapid enlargement of the underground channel, and eventually 
the entire stream followed this course. 
Subsequent erosion and caving have greatly decreased the 
length of the underground channel and produced the short 
cation above the present entrance portal. Owing to the thin- 
ness of the roof, the first part of the tunnel caved rapidly, 
whereas the erosion and caving of the lower part was much 
slower. A condition now has been reached where the entrance 
and exit portals will be eroded with about equal speed and at 
the same time the ttmnel will be greatly enlarged in diameter 
by down cutting and by caving of the roof. Under these con- 
ditions the bridge eventually should assume the arch structure 
characteristic of famous natural bridges in other parts of the 
world. ; 
PAUL R. FANNING. 
1188216 
