TUN INDIANA CAVES. 101 



ing action of rain or carbonated water upon the lime- 

 stone strata in which they occur. 



Carbon dioxide is present everywhere in the atmos- 

 phere, constituting about three parts in 10,000 of the 

 volume thereof. The condensed vapors, falling as 

 rain, unite with a portion of this carbon dioxide, and 

 form a weak carbonic acid or rain-water. Wherever 

 this comes in contact with limestone, 



Formation ^ ^^^ about a chemical change. 

 of Sink-holes . & 



and Caves. **J this change the limestone is dis- 

 solved and carried onward with the 

 seeping or flowing waters. 



In the beginning of a sink-hole, the rain-water 

 seeps through a crevice or joint of the limestone to a 

 lower stratum, along the surface of which it finds a 

 passage. By gradually dissolving the stone, this 

 passage-way becomes increased, until finally a large 

 cavity is formed immediately below the surface. The 

 unsupported weight of the latter causes it to grad- 

 ually sink downward and assume the inverted cone 

 shape above mentioned. The opening at the bottom 

 becomes larger, allows more water to enter, and a 

 more rapid dissolving takes place between the layers. 

 As soon as the under-ground passage has become large 

 enough to allow a good-sized stream to enter, the 

 process of erosion or abrasion is added to that of the 

 solvent action of the water and the enlargement of the 

 passage goes on much more rapidly. This gradual 

 enlargement continues for hundreds, perhaps thou- 

 sands, of years and results in a cave, varying in size 

 according to its age, the amount of water flowing 

 through it and the softness of the rock dissolved or 



