in the two hemispheres; the family of Fagoaga, or of the Marquis del Apartado, drew from thence in a few months a neat profit of 870,000. The produce of the mine of Valenciana, worked in transition slate, has been so constant, that to the end of the last century it never ceased to furnish anually, during forty years successively, above 222,000lbs. troy of silver. In general, in the central part of New Spain, where porphyries are frequent, it is not that rock which affords the precious metals in the three great workings of Guanaxuato, Zacatecas, and Catorce. These three mining districts, which yield the half of all the Mexican gold and silver, are situated between the 18th and 23d degrees of north latitude. The miners there work on metalliferous mineral deposits, almost entirely in intermediary formations of clay-slate, grauwacke, and alpine limestone; I say almost entirely, for the famous Veta madre de Guanaxuato, richer than Potosi, and furnishing till 1804, on an average, a sixth of the silver which America pours into the circulation of the whole world, traverses both clay-slate and porphyry. The mines of Belgrado, San Bruno, and Marisanshez, opened in the porphyritic part at the south-east of Valenciana, are but of small importance. Other workings carried on in the porphyries of Real del Monte, Moran, Pachuca, and Bolaños, do not now furnish above 62,000lbs. troy, or a twenty-fifth part of the silver exported (1803) from the port of Vera Cruz. I thought it proper here to state these GEOLOGY OF MINING DISTRICTS. 141 facts, because the denomination of metalliferous porphyries which I have often used in my works, might lead to the error of considering the metallic riches of the New World as procured in great part from transition porphyries. The more we advance in the study of the constitution of the globe in different climates, the more we are convinced that there scarcely exists one rock anterior to mountain limestone which has not been found in some countries extremely argentiferous. The phenomenon of these ancient veins in which our metallic riches are deposited, (perhaps as the specular iron and muriate of copper are deposited in modern times in the fissures of lava,) is a phenomenon that appears in some degree independent of the specific nature of rocks. In advancing to the north towards Sopilote, Mescala, and Tasco, we again lose sight of porphyry. Primitive granite re-appears, but is soon hid by a porphyry, the mineralogical composition of which presents very remarkable characters; it is blueishgray, a little argillaceous by decomposition, and contains large crystals of whitish-yellow feldspar (rather lamellar than glassy), pyroxene of nearly a leek-green, and a little uncrystallized quartz. This stratified porphyry is covered, towards the south, with the same conglomerate limestone that abounds on the table-land of Chilpansingo; towards the north (Sopilote, Estola, Mescala) with a grayish compact limestone traversed by veins of carburet ted lime. The limestone of Estola is not always spongy or vesicular in its whole mass, like the formation of Masatlan, but contains large insulated caverns like the limestone of Peregrino. Whilst travelling in those mountains, I had no doubt remaining, that the rocks of Cañada de Sopilote and of Alto del Peregrino are identical with our alpine limestone (zechstein) of Europe, which succeeds, according to the age of its formation, to the red sandstone, or, when that is wanting, to the transition rocks. Near Mescala, a little north of Sopilote, rich silver veins, analogous to the veins of Tasco and Tehuilotepec, traverse the alpine limestone. The rock in the valley of Sopilote, that covers the porphyry of the group of Zumpango, exhibits the same sinuous and contorted beds that are seen at Achsenberg, on the bank of the lake of Lucerne, and in other mountains of alpine limestone in Switzerland. I observed that the upper beds of the formation of Sopilote and Mescala passed progressively to whitish-gray, were destitute of veins of calcareous spar, and presented a dull, compact, or conchoidal fracture. They divide, nearly like the limestone of Pappenheim, into very thin layers. It seems to be the passage from the alpine to the Jura limestone, two formations which immediately cover each other in Switzerland, the Apennines, and several parts of equinoctial America, but which in the south of Germany are separated from each other GEOLOGY OF MINING DISTRICTS. 143 by several interposed formations (by the sandstone of Nebra, or bunte sandstein, by muschelkalk, and the white sandstone or quadersandstein). Near the village of Sochipala, the mountain limestone is covered by gypsum, and between Estola and Tepecuacuilco, there appears beneath the mountain limestone (directed sometimes N. 48° E., with inclination of 40° at the east; sometimes N. 48° E.; with inclination 50° to south-east) a porphyry, asparagus-green, with a base of compact feldspar, divided into very thin strata like that of Achichintla, and almost destitute of disseminated crystals. This rock reseinbles phonolitic porphyry (porphyr-schiefer) of the trachyte formation. In advancing towards the mines of Tehuilotepec and Tasco, we find the same rock covered with a quartzose sandstone having a cement of argillaceous limestone, and analogous to the weiss liegende (lower arenaceous bed of zechstein) of Thuringia. This quartzose sandstone again announces the proximity of mountain limestone; and in fact, on this sandstone and perhaps immediately on porphyry (as at Zumpango, and the Alto de los Caxones), near the salt lake of Tuspa, an immense mass of mountain limestone reposes, often cavernous, and containing some petrifactions of trochi and other univalve shells. This limestone of Tuspa, indubitably posterior to all the porphyries which I have just described, contains beds of specular gypsum, and beds of slaty and carburetted clay, which we must not confound with grauwacke-slate. It is generally blueish-gray, compact and crossed by veins of carbonate of lime. In many places, instead of being cavernous, it passes to a very compact white formation, analogous to the limestone of Pappenheim. I was struck with these variations of texture, which M. de Buch and myself had also observed in the Apennines (between Fosombrono, Furli, and Fuligno), and which seem to prove that, where the intermediary members of the series were not developed, the formations of alpine and Jura limestone are more closely connected than is generally admitted. The rich silver veins of Tasco, which formerly yielded 99,000lbs. troy of silver annually, traverse both limestone and a clayslate that passes to mica-slate; for, notwithstanding the identity of the limestone formations of Tasco and Mescala, which are both argentiferous, the former of those, wherever it has been pierced in mining (Cerro de S..Ignacio), has not been found superposed to porphyry like the limestone of Mescala, but covering a more ancient rock than porphyry, a mica-slate (dir. N. 50° E.; incl. 40°-60°, most frequently at N. W. sometimes at S. E.) destitute of garnets, and passing to primitive clay-slate. It was proper to enter into these details on the formations that succeed porphyries, because it is only in making known the nature of superposed rocks, that geognosts can be enabled to decide on the place which Mexican porphyries ought to occupy in the order of formations. A sketch of a geognostic ta |