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Gigante belongs to alluvious lands, and the veins
of tin of the Sierra de Guanaxuato are found in mountains of porphyry. In the mines of Co
manja, a syenite apparently of antient formation
contains a seam of silver; the mine of Guamaxuato the richest of all America crosses a primitive slate (thon-schiefer) which frequently passes into the talk-slate (talk-sehiefer): the serpentine of Zimapan appears destitute of metals. The porphyries of Mexico may be considered for the most part as rocks eminently rich in mines of gold and silver. One of the problems of geology the most difficult to resolve is the determination of their relative antiquity. They are all characterised by the constant presence of amphibole and the absence of quartz, so common in the primitive porphyries of Europe, and especially in those which form beds in gneiss. The common felspar is rarely to be seen in the Mexican porphyries; and it belongs only to the most antient formations, those of Pachuca, Real del Monte and Moran, where the veins furnish twice as much silver as all Saxony. We frequently discover only vitreous felspar in the porphyries of Spanish America. The rock which is intersected by the rich gold vein of Villalpando near Guanaxuato is a porphyry of which the basis is somewhat a kin to klingstein (phonolite), and
in which amphibole is extremely rare. Several of these parts of New Spain bear a great analogy to the problematical rocks of Hungary, designated by M. Born by the very vague denomination of saatum metailiferum. The veins of Zimapan which are the most instructive in respeet to the theory of the stratification of minerals are intersected by porphyries of a greenstone base which appear to belong to trap rocks of new formation. These veins of Zimapan offer to oryctognostic collections a great variety of interesting minerals such as the fibrous zeolith, the stilbite, the grammalite, the pycnite, native sulphur, spar fluor, baryte suberiform asbestos, green grenats, carbonate and chromate of lead, orpiment, chrysoprase, and a new species of opal of the rarest beauty, which I made known in Europe, and which M. M. Karsten and Klaproth have described under the name of (feuer-opal.)
. Among the transition rocks which contain silver minerals, we may mention the transitionlime-stone (ibergangs-kalkstein) of the Real del Cardonal, of Xacala and of Lomo del Toro, to the north of Zimapan. In the last of these places what is worked is not veins but masses of galena, of which some nests have yielded in a short space of time according to the observation of M. Sonneschmidt, more than 124,000 quintals of lead. The graumakke alternating with the graunakken slate is equally rich in metals in Mexico as in several parts of Germany. In this rock the formation of which immediately preceded that of the secondary rocks, several of the veins of Zacatecas appear to be found. •. -
In proportion as the north of Mexico shall be examined by intelligent geologists, it will be perceived that the metallick wealth of Mexico does not exclusively belong to primitive earths and mountains of transition, but extend also to those of secondary formation. I know not whether the lead which is procured in the eastern parts of the intendancy of San Luis Potosi is found in veins or beds, but it appears certain, that the veins of silver of the real de Catorce, as well as those of the Doctor and Xaschi near Zimapan, traverse the alpine lime-stone (alpenkalkstein); and this rock reposes on a poudingue with silicious cement which may be considered as the most antient of secondary formations. The alpine lime-stone and the jura lime-stone (jurakalkstein) contain the celebrated silver mines of Tasco and Teuilotepec in the intendancy of Mexico; and it is in these calcareous rocks that the numerous veins which in this country have been very early wrought, display the greatest wealth. They are more sterile in the strata of primitive slate (ur-thon-schiefer) which as is seen in the Cerro de San Ignacio, serves for base to the secondary formations. The result of this general view of the metalliferous depositories (erzführende lagerstätte) is that the cordilleras of Mexico contain veins in a great variety of rocks, and that those rocks which at present furnish almost the whole silver annually exported from Vera Cruz, are the primitive slate, the graunakke, and the alpine lime-stone, intersected by the principal veins of Guanaxuato, Zacatecas and Catorce. Thus it is in a primitive slate (ur-thon schiefer) on which a clayey porphyry containing grenats reposes, that the wealth of Potosi in the kingdom of Buenos-Ayres is contained. On the other hand, in Peru the mines of Gualgayoc or Chota and that of Yauricocha or Pasco which together yield annually double the quantity of all the German mines, are found in an alpine limestone. The more we study the geological constitution of the globe on a large scale the more we perceive that there is scarcely a rock which has not in certain countries been found eminently metalliferous. The wealth of the veins is for the most part totally independent of the nature of the beds which they intersect. We observe in the most celebrated mines of Europe, that the mining operations are either directed to a multitude of 'small veins as in the primitive mountains of Saxony, or to a very
small number of depositories of minerals of an extraordinary power, as at Clausthal, the Harz, and near Schemnitz in Hungary. The cordilleras of Mexico offer frequent examples of these two methods of operation; but the districts of mines of the most constant and considerable wealth, Guanaxuato, Zacatecas and the Real del Monte, contain only one principal vein each (veta madre). The vein called halsbrükner spath of which the extent is two metres" and which has been traced for a length of 6200 metrest is spoken of as a remarkable phenomenon at Freiberg. The veta madre of Guanaxuato, from which there has been extracted during the course of the last ten years more than six millions of marcs of silveri, is of the extent of from 40 to 45 metress, and it is wrought from Santa Isabella and San Bruno to Buena-Vista, a length of more than 12700 metres||. In the Old Continent, the veins of Freiberg and Clausthal which intersect mountains of gneiss and graumakke are visible in table lands of which the elevation above the level of the sea is only from 350 to 570 metres;" and this • 6 feet. Trans. + 20,341 feet. Trans. £ 3,937,899 lb troy. Trans. § From 131 to 147 feet. Trans.
|| 41,665 feet. Trans. * From 1148 to 18694 eet. Trans.