I have just found the passage I referred to the other day, relative to the existence of native or sandrock nitre in the interior of Southern Africa. It is in Barrow, and not in Vaillant, as I thought when I had the pleasure of conversing with you concerning it. I am much obliged to you for recalling my attention to that curious subject, as it has brought to my recollection a fact, which I believe I omitted to mention in my memoir, (viz.) the existence of a black substance in the clay under the rocks, of a bituminous appearance and smell. This I remember to have seen in a rock-house, near the Kentucky river, where very considerable quantities of sandrock nitre had been obtained. This substance was found in masses of a few ounces weight, and in the crevices of the rocks near the basis of the side walls. The smell was not wholly bituminous, but resembled that of bitumen combined with musk. I am quite unable to account for the formation of the nitre, or the production of this black substance which sometimes accompanies it, both in Africa and America. Had I seen Mr. Barrow's travels, when I noticed the bitumen, I should certainly have paid more attention to it. But perceiving no relation between the rock nitre and the masses of this substance, my examination of it was much too superficial. I do not very well understand what Mr. Barrow means by saying, that many wagon loads of animal matter lay on the _roof_ of the caverns in Africa. I saw no such matter on the _roof_ of the rock-houses in Kentucky. Certainly the caverns have been the habitations of wild beasts, and great quantities of leaves, &c. have been mixed with the debris of the superincumbent rocks, but it does not seem probable, that much animal matter could be filtrated through a roof of rock, perhaps forty or fifty feet in thickness. The subject, however, is very curious, and deserves much more attention than any of us have bestowed upon it. _Extract from Barrow's Southern Africa, p. 291. New-York edition._