26 GEOLOGICAL STRUCTURE OK THE 



position, or a mere adventitious production formed 

 by the partial agency of the decomposed pyrites so 

 prevalent in the argillaceous bed above noticed, I 

 am not prepared to ascertain. It occurred in seams, 

 though divided into small and rounded masses, per- 

 fectly white, but so devoid of the fibrous structure as 

 to be readily confounded with the chalk. The si- 

 milarity of this secondary calcareous formation on 

 its opposite confines in East Tennessee, as it appears 

 immediately after crossing the Cumberland gap is 

 deserving of attention; here again the calcareous 

 rock puts on the appearance of chalk, and even con- 

 tains nodules of flint, but bordering too much on chal- 

 cedony to afford the character requisite for economi- 

 cal purposes. 



Before taking leave of this part of our subject, and 

 indeed not unconnected with it, is the anomalous de- 

 position of salt, and the production of nitre. We all 

 know that the impure nitre of the western states, of 

 which the greatest abundance has been found in the 

 neighbourhood of the Cumberland ridge of mountains 

 on the confines of East Tennessee, is always connect- 

 ed with the caverns of calcareous and arenilitic rock, 

 and that it is not an accidental production, arising from 

 the decomposition of animal and vegetable matter, is 

 indeed proved by its gradual renewal in those caverns 

 which have been exhausted. As 1 have been inform- 

 ed, it exists in the calcareous and sandstone rocks 

 which arc consequently attacked by the humidity of 

 the air, and so falls into earthy fragments, which are 



