When, in 1785, after an earthquake, a great rocky mass was broken off in the Aroyo del Robalo, the Guaykeries of Los Serritos collected fragments of alum five or six inches in diameter, extremely pure and transparent. It was sold in my time at Cumana to the dyers and tanners, at the price of two reals* per pound, while alum from Spain cost twelve reals. (* The real is about 6 1/2 English pence.) This difference of price was more the result of prejudice and of the impediments to trade, than of the inferior quality of the alum of the country, which is fit for use without undergoing any purification. It is also found in the chain of mica-slate and clay-slate, on the north-west coast of the island of Trinidad, at Margareta and near Cape Chuparuparu, north of the Cerro del Distiladero.* (* Another place was mentioned to us, west of Bordones, the Puerto Escondido. But that coast appeared to me to be wholly calcareous; and I cannot conceive where could be the situation of ampelite and native alum on this point. Was it in the beds of slaty clay that alternate with the alpine limestone of Cumanacoa? Fibrous alum is found in Europe only in formations posterior to those of transition, in lignites and other tertiary formations belonging to the lignites.) The Indians, who are naturally addicted to concealment, are not inclined to make known the spots whence they obtain native alum; but it must be abundant, for I have seen very considerable quantities of it in their possession at a time. South America at present receives its alum from Europe, as Europe in its turn received it from the natives of Asia previous to the fifteenth century. Mineralogists, before my travels, knew no substances which, without addition, calcined or not calcined, could directly yield alum (sulphate of alumina and potash), except rocks of trachytic formation, and small veins traversing beds of lignite and bituminous wood. Both these substances, so different in their origin, contain all that constitutes alum, that is to say, alumina, sulphuric acid and potash. The ores of Tolfa, Milo and Nipoligo; those of Montione, in which silica does not accompany the alumina; the siliceous breccia of Mont Dore, which contains sulphur in its cavities; the alumiferous rocks of Parad and Beregh in Hungary, which belong also to trachytic and pumice conglomerates, may no doubt be traced to the penetration of sulphurous acid vapours. They are the products of a feeble and prolonged volcanic action, as may be easily ascertained in the solfataras of Puzzuoli and the Peak of Teneriffe. The alumite of Tolfa, which, since my return to Europe, I have examined on the spot, conjointly with Gay-Lussac, has, by its oryctognostic characters and its chemical composition, a considerable affinity to compact feldspar, which constitutes the basis of so many trachytes and transition-porphyries. It is a siliciferous subsulphate of alumina and potash, a compact feldspar, with the addition of