THE MAMMOTH CAVE. 51 



never entirely destitute of nitrifiable matter. 

 The nitric acid which enters into its composition 

 may, in some measure, be derived from the at- 

 mosphere. The kind of nitre that is found in 

 the Cave is the nitrate of lime, which, when re- 

 acted upon by the carbonate of potash, is trans- 

 formed into the nitrate of potash, or common 

 saltpetre. This was the course pursued by the 

 saltpetre miners when that substance was manu- 

 factured in the Cave in 1812-14. The nitrate of 

 lime is found in the dryer parts of the Cave, 

 but is not discoverable till the earth which con- 

 tains it is lixiviated. 



" The mechanical agencies concerned in the 

 excavation of the Mammoth Cave are trifling 

 when compared with the chemical. They are 

 instanced in the transportation of gravel, sand, 

 and clay from one part of the Cave to another, 

 and in the abraded appearances presented by 

 the rock composing certain avenues. Thus, it is 

 possible to tell the direction in which the water 

 ran in most of the avenues, and the rapidity of 

 its motion, by observing the points at which 

 gravel, sand, and clay are deposited, and the 

 order in which they come. For example, the 

 points at which gravel is deposited indicate a 

 rapid current; where sand is found, the move- 



