Dr. Brown, in _Silliman's Journal_, i, p. 147, in a letter to professor Silliman, observes, that there exists a black substance in the clay under the rocks, of a bituminous appearance and smell. This black substance, it appears, accompanies the sand-rock nitre, and is the same as that found in Africa, which also accompanies nitre in that country. Animal matter seems to have existed in the nitre caves of Africa, forming, as Mr. Barrow expresses it, either a _roof_ or covering; no such matter, however, has ever been found in or adjacent to the nitre caves of the Western country. The observations of Mr. Barrow on the subject of the saltpetre of Africa may be interesting to the reader. He observes, (_Southern Africa_, p. 291,) that, about twelve miles to the eastward of the wells, (_Hepatic Wells_), in a kloof of the mountain, we found a considerable quantity of native nitre. It was in a cavern similar to those used by the Bosgesmans for their winter habitations. The _under surface_ of the projecting stratum of calcareous stone, and the sides that supported it, were incrusted with a coating of _clear, white saltpetre_, that came off in flakes. The fracture resembled that of refined sugar; it burnt completely without leaving any residuum; and if dissolved in water, and thus evaporated, crystals of pure _prismatic nitre_ were obtained. This salt, in the same state, is to be met with under the sand-stone strata of many of the mountains of Africa. There was also in the same cave, running down the sides of the rock, a black substance, that was apparently bituminous. The peasants called it the urine of the das. The dung of this _gregarious_ animal was lying upon the roof of the cavern to the amount of many wagon loads. 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.