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Geological constitution of mines-position and extent of
mineral veins-elevation of mines—comparative wealth of mines—distance between principal mines-nature of ores—mean riches of ores.
As the subject of the disposition of the mineral treasures of Mexico in the bosom of the earththe direction, inclination, and character of the veins, is of primary importance, we shall communicate the result of some further observations on those interesting phenomena, together with some details as to the comparative wealth of the most celebrated veins.
Those who have studied the geological constitution of a mining country of great extent, know the difficulty of reducing to general ideas the observations made on a great variety of beds and metalliferous veins. These difficulties are increased when it happens, as in the mountains of Mexico, that the veins, the beds, and the masses (stockwerke), are scattered in an infinity of mixed rocks of very different formations. If we possessed an accurate description of the four or five thousand veins actually wrought in Mexico, or which have been wrought within the two last centuries, we
should undoubtedly perceive, in the materials and structure of these veins, analogies indicative of a simultaneous origin; we should find that the matters comprising the veins are partly the same with those which are exhibited in the veins of Saxony and Hungary, and on which M. Werner, the first mineralogist of the age, has thrown so much light. But we are yet very far from being acquainted with the metalliferous mountains of Mexico; and notwithstanding the great number of observations collected by myself in travelling through the country in different directions, for a length of more than 1200 miles, I shall not venture to sketch a general view of the Mexican mines, considered under their geological relations; I shall content myself with merely indicating the rocks which yield the greatest part of the wealth of New Spain.'
In the present state of the country, the veins or lodes are the object of the most considerable operations; and the ores disposed in beds or in masses are not frequent. The Mexican lodes are for the most part found in primitive and transition rocks, and rarely in the rocks of secondary formation.
On taking a general view of the metalliferous repositories, we find 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,
primitive slate or killas, grauwacke, and alpine limestone, intersected by the principal veins of Guanaxuato, Zacatecas, and Catorce. : :
· The porphyries of Mexico may be considered for the most part as rocks eminently rich in ores of gold and silver. 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.
In proportion as the north of Mexico shall be examined by intelligent geologists, it will be perceived that the metallic wealth of Mexico does not exclusively belong to primitive formations and transition rocks, but extends 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 alpine limestone ; and this rock rests on puddingstone, with siliceous cement, which may be con sidered as the most ancient of secondary formations. The alpine limestone, and the jura limestone, contain the celebrated silver mines of Tasco and Tehuilotepec, 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. Mines in Europe are usually worked either on a great number of small veins, or on fewer depositories of minerals of great size, 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 halsbruckner spath, of which the extent is 67 feet, and which has been traced for a length of 339 fathoms, is spoken of as a remarkable phenomenon at Freiberg. The veta madre of Guanaxuato, from which there have been extracted, during the course of the last ten years, nearly 4,000,000lbs. troy of silver, is of the extent of from 130 to 150 feet, and it is wrought from Santa Isabella and San Bruno to Buena-Vista, a length of more than 6944 fathoms.
In the New Continent the metallic wealth is deposited by nature on the very ridge of the Cordilleras, and sometimes in situations within a very small distance from the limit of perpetual snow. The most celebrated mines in Mexico are at absolute heights of from 5900 to 9840 feet. -
We have mentioned in another place the advantage which, in working the Mexican mines, is derived from the most important veins being in a
middle region, where the climate is not unfavourable to agriculture and vegetation. The large town of Guanaxuato is placed in a ravine, the bottom of which is somewhat lower than the level of the lakes of the valley of Tenochtitlan. We are ignorant of the absolute heights of Zacatecas and the Real de Catorce; but these two places are situated on table-lands seemingly more elevated than the level of Guanaxuato. However, the temperate climate of these Mexican towns, which are surrounded with the richest mines in the world, is a contrast to the cold and exceedingly disagreeable climate of Micuipampa, Pasco, Huancavelica, and other Peruvian towns. .
The 1,544,000lbs. of silver which are annually sent to Europe and Asia, from the ports of Vera Cruz and Acapulco, are the produce of a very small number of mines. The three districts which we have frequently had occasion to name, Guanaxuato, Zacatecas, and Catorce, supply more than the half of that sum. The vein of Guanaxuato alone yields more than a fourth part of the whole silvero Mexico, and a sixth part of the produce of all America.
The following is the order in which the richest mines of New Spain follow one another, arranging them according to the quantity of money actually drawn from them:
Guanaxuato, in the Intendancy of the same