SUEFACE FEATUEES. 81 



the heat of the summer sun upon the outer wall of the fissure 

 and valley -side. This outer wall is from ten to twenty feet in 

 thickness where the ice was seen to be most abundant. The 

 water for its production seems to be supplied by slow exuda- 

 tion from the inner wall of the cave. 



Ice caverns, more extensive than this are known in various 

 parts of the world, but this is, so far as now known, the only 

 one in Iowa, and has consequently excited much popular 

 interest. 



11 



