Itimestone. 15 



as that of the small streams and rivers. Pure 

 water is a powerful solvent. When the rains 

 fall upon the earth the water percolates 

 through it and through the limestone strata, 

 which gradually wears away the limestone and 

 carries it back to the ocean, so that the process 

 of tearing down and building up is continually 

 going on. The great caves that are found 

 everywhere in the limestone regions were 

 formed by the action of water. The great 

 Mammoth Cave of Kentucky, which is said to 

 have 200 miles of underground passages, has 

 been entirely worn out by the action of run- 

 ning water. 



Some years ago the writer visited this cave 

 and had an opportunity to study the wonder- 

 ful eroding or gnawing-out effect of water on 

 limestone. At some period earlier in the his- 

 tory of the earth there was evidently an under- 

 ground river or large stream of water that 

 found its way through the crevices of the 

 rocks, and gradually wore out a great bed for 

 itself, which was fed by lateral streams pour- 

 ing into the main branch, each one of which 

 lateral branches cut its own channel. A plan 

 view of the Mammoth Cave presents a picture 

 not unlike that of a great river with numerous 

 branches emptying into it, all of them showing 

 the windings such as we see in a river and its 

 feeders upon the surface of the earth. There 

 are three sets of these channels, one above the 



