change in the price of commodities in Europe, has been extracted from that part of the vein alone contained between the pits of Esperanza and Santa Anita, an extent of less than 2600 metres*. In this part we find the mines of Valenciana, Tepeyac, Cata, San Lorenzo, Animas, Mellado, Fraustros, Rayas, and Santa Anita, which at different periods have been very highly celebrated. The veta madre of Guanaxuato, bears a good deal of resemblance to the celebrated vein of Spital of Schemnitz, in Hungary. The European miners who have had occasion to examine both these depositories of minerals, have been in doubt whether to consider them as true veins, or as metalliferous beds (erzlager). If we examine only the veta madre of Guanaxuato, where the roof and the wall in the mines of Valenciana or Rayas, are of clay slate, we might be tempted to acquiesce in the latter opinion; for far from cutting or crossing the strata of the rock (quergestein), the veta has exactly the same direction and the same inclination as its strata; but can a metalliferous bed which has been formed at the same period, as the whole mass of the mountain in which it is to be found, pass from a superior to an inferior rock, from porphyry to clay slate? If the veta madre was really a bed, * 8529 feet. Trans. we should not find angular fragments of its roof contained in its mass, as we generally observe on points where the roof is a slate charged with carbone, and the wall a talc slate. In a vein, the roof and the wall are deemed anterior to the formation of the crevice, and to the minerals which have successfully filled it; but a bed has undoubtedly pre-existed to the strata of the rock which compose its roof. Hence we may discover in a bed fragments of the wall, but never pieces detached from the roof. The veta madre of Guanaxuato, exhibits the extraordinary example of a crevice formed according to the direction and inclination of the strata of the rock. Towards the south east from the ravin of Serena, or from the mines of Belgrado and San Bruno, which are very fully wrought, to beyond the mines of Marisanchez, it runs through porphyritic mountains; and towards the north east on leaving the pits of Guanaxuato, to the Cerro de Buena Vista, and the Cañada de la Virgen, it traverses the clay slate (thon scheifer). Its extent varies like that of all the veins of Europe. * M. Werner in the Theory of Veins, 2. expressly says, "that the depositories of minerals almost always cut the "banks of the rock." This great mineralogist seems to have intended to indicate by these words, that there may be true veins parallel to the folia of a clay, or micaceous slate. When not ramified it is generally from 12 to 15 metres* in breadth; sometimes it is even strangled to the extent of half a metre‡; and it is for the most part found divided into three masses, (cuerpos) separated either by banks of rock, (caballos) or by parts of the gangue almost destitute of metals. In the mine of Valenciana the veta madre has been found without ramification, and of the breadth of 7 metres§, from the surface of the ground to the depth of 170 metres. At this point it divides into three branches, and its extent, reckoning from the wall to the roof of the entire mass, is 50 and sometimes even 60 metres. Of these three branches of vein, there is in general but one alone which is rich in metals; and sometimes when all the three join and drag one another, as at Valenciana near the pit of San Antonio, at a depth of 300 metres**, the vein contains immense riches on an extent (puissance) of more than 25 metrestt. In the pertinencia de Santa Leocadia, four branches are observable. *From 38 to 48 feet. Trans. + At the place of assemblage of the pit of Santo Christo de Burgos, in the Mine of Valenciana. A trum of which the inclination is 65° separates from the inferior branch, (cuerpo baxo) and cuts the folia of the rock of the wall. This phenomenon, and the great number of druses, abounding with amethyst chrystals, to be found in the mines of Rayas, which affect the most different directions, are sufficient to prove that the veta madre is a vein, and not a bed. Other proofs not less convincing might be drawn from the existence of a vein, (veta del caliche) wrought in the compact limestone of Animas, which is parallel to the principal vein of Guanaxuato, and has exhibited the same silver minerals. Is this identity of formation ever found between two metalliferous beds, which belong to rocks of very different antiquity? The small ravins into which the valley of Marfil is divided, appear to have a decided influence on the richness of the veta madre of Guanaxuato, which has yielded the most metal where the direction of ravins, and the slope of the mountains, (flaqueza del Cerro) have been parallel to the direction and inclination of the vein. When we stand on the elevation of Mellado, near the pit which was dug in 1558, we observe that the veta madre is in general most abundant in minerals towards the north west, towards the mines of Cata and Valenciana; and that to the south east towards Rayas and Santa Anita, the produce has been at once richer, rarer, and more inconstant. Besides in this celebrated vein, there is a certain middle region which may be considered as a depositary of great riches; for above and below this region, the minerals have contained an inconsiderable share of silver. At Valenciana the rich minerals have been in the greatest abundance, between 100 and 340 metres* in depth below the mouth of the galery. This abundance appeared at Rayas at the surface of the earth; but the galery of Valenciana is pierced according to my measurements†, in a plain which is more than 156 metres above the level (galerie d'ecoulement) of Rayas; which might lead us to believe that the depository of the great wealth of Guanaxuato is found in this part of the vein, between 2130 and 1890 metres of absolute height above the level of the oceans. The deepest works of the mine of Rayas, (los planes) have never yet reached the inferior limit of this middle region; while the bottom (das tiefste) of the mine of Valenciana, the galery of San Bernardo has unfortunately passed this limit more than 70 metres. Hence * Between 328 and 1115 feet. Trans. + See my Recueil d'Observations Astronomiques, Vol. i. p. 324. No. 332-357. 511 feet. Trans. Between 6987 and 6199 feet. Trans. |