The Rev. Mr. Cornelius, in describing a cave in the Cherokee country at Nicojack, the north west angle in the map of Georgia, (_Silliman's Journal_, vol. i, p. 321,) observes, that it abounds with nitrate of potassa, a circumstance very common to the caves of the Western country, and is found covering the surfaces of fallen rocks, but in more abundance beneath them. There are two kinds; one is called the "clay dirt," the other the "black dirt." The earth, however, contains calcareous nitre, and for that reason an alkaline lixivium is employed. In short, the process employed there is the same as at the other saltpetre caves which we have described. One bushel of the clay dirt yields from three to five pounds of nitre, and the black dirt from seven to ten pounds. It seems also, that the same dirt, if carried back to the cave, will become impregnated with nitre.
Mr. Cornelius remarks, that these caves have been used by the natives as burial places; in one of which he counted a hundred human skulls in the space of twenty feet square; and infers, that, by the decomposition of animal matter, the acid of nitric salts arises, and therefore that this may have occasioned the formation of the nitrates of potassa and lime.
At Corydon, in Indiana, there is a cave, which, according to Stilson's account, contains both nitrate of lime, and nitrate of magnesia. It is not worked.
Kain, in his remarks on the Geology and Mineralogy of East Tennessee, (_Silliman's Journal_, vol. i, p. 65,) observes, that the numerous caves which have been found in the Cumberland mountains, and other parts of Tennessee have been very productive of nitrate of potassa; and in confirmation of the remarks before made, he adds, in investigating the causes that have given rise to these salts, that wild animals burrow in these caves; that, when pursued by the hunter, they make them the places of their retreat, and probably die there; that the aborigines have made them a place of burial; and that the streams of water, which flow through them, in wet weather, carry with them not only great quantities of leaves, but many other vegetable productions.
Without offering any theory, by which we may account for the formation of nitre, in nitre caves, or in situations which cannot be influenced by the putrefactive process, we may merely remark, that as nitric acid is composed of oxygen and azote, there must be some operation unknown to us, by which the union of these elements takes place. Nascent azote must unite with the base of oxygen gas; but whence, in saltpetre caves, proceeds the azote and the oxygen? It appears that calcareous bodies facilitate the formation of nitre, as they do in artificial nitre beds. The greater part of the nitrous earth is lime; and it also appears, that the same earth, after the extraction of the saltpetre, will again furnish it. We know that lime is a compound of a base called calcium united with oxygen; but in what manner it promotes the union of azote and oxygen, or furnishes either one or the other of these bodies, or perhaps both, is altogether uncertain. Nor can we account for the formation of potash in the native nitre of the nitre caves. In other situations, as for instance where nitrous efflorescence appears on the earth, and in artificial nitre beds, in which animal and vegetable substances are in the act of decomposition by the putrefactive fermentation, we may account for the generation of nitric acid.