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I have specified in these tables, year' after year, the gold and silver extracted from the mines of Guanaxuato from 1766 to 1803; and I have distinguished the metals procured from the ores by means of amalgamation, from those obtained by smelting. A marc of gold contains 50 castellanos, which are equal to 400 tomines, or 4800 granos. The result of these tables, which are framed from official papers *, is that the district of mines of Guanaxuato has pro. duced in 38 years gold and silver to the value of 165 millions of piastres t; and that from 1786 to 1803, the annual average produce has been 556,000 marcs of silver f, equal to 4,727,000 piastres. All the veins of Hungary and Transylvania together, only yield 85,000 marcs of silver. S
Taking four averages of years, of which three are of five and one of eight years, we shall have the following results:
• Razon de los Castellanos de oro de ley 22 quilates y marcos de plata, de 12 dineros de los beneficios de Azogue y Fuego, manifestados en la tresoreria principal de Real Hacienda de Guanaxuato, desde 10 de Enero 1766 hasta 31 de Deciembre 1803. (Manuscript.) We have computed the marc of silver at 84 piastres, and the marc of gold at 136 piastres (the piastre being equal to 5 livres 5 sous).
+ 12,720,061 lb. Troy. 'Trans,
What is the nature of the metalliferous depository, which has furnished these immense riches; and which may be considered as the Potosi of the northern hemisphere. What is the position of the rock which crosses the veins of Guanaxuato ? These questions are of so great importance that I must here give a geological view of so remarkable a country.
The most ancient rock known in the district of Guanaxuato, is clay slate (thon-schiefer) which rests on the granite rocks of Zacatecas and the Peñon Blanco. * It is of an ash-grey or greyish black color, frequently intersected † by an infinity of small quartz veins, and at a considerable depth passes into talk-slate (talk-schiefer) and into schistous chlorite. I consider this clay-slate as of primitive formation, although
* Sonneschmidt, Beschreibung der Bergwerks-Refiere von Mexico, p. 194 & 292.
† In the quebrada of San Roquito, which communicates with the ravin of Acabuca.
the beds with very thin folia which it contains, and which are surcharged with carbon, appear to approximate it to transition clay slate. These beds (oja de libro) are for the most part found near the surface * ; but sometimes they are visible t at considerable depths. On digging the great pit (tiro general) of the Valenciana, they discovered banks of syenite; of hornblend slate (hornblend schiefer) and true serpentine, alternating with one another, and ferming subordinate beds, in the clay slate. This extraordinary phenomenon of a syenite alternating with serpentine, is also to be seen in the island of Cuba, near the village of Regla, where the latter rock abounds in schillerspar (schillerspath). The same clay slate of Guanaxuato which is observed at the bottom of the mine of Valenciana, re-appears at the surface, eight hundred metres # higher up on the ridge of the Sierra de Santa Rosa, but I doubt whether it has ever been found at greater elevations. These strata are very regularly directed h. 8 to 9. of the miner's compass $; they are inclined from 45 to 50
* In the Valenciana mine.
§ Or from South-East to North-West. I have been struck ever since 1791, with this great law of the parallelism of the · strata, which are discovered in immense extents of country, and which may be regarded as one of the most curious phenoVOL, III.
degrees to the south-west. This is the direction of the greatest part of the very old rocks of Mexico.
Two very different formations rest on the clay slate : the one of porphyry at considerable elevations to the east of the valley of Mar. fil, and to the north-west of Valenciana ; and the other, of old sandstone in the ravins, and table lands of small elevation.
Porphyry forms gigantic stony masses, which appear at a distance, under the strangest aspect, frequently like ruins of walls and bastions, These masses are perpendicular, and from three to four hundred metres *, elevated above the
mena of geology. I have never ceased in my writings from calling the attention of travellers to an object, with regard to which it would be easy to collect in a very short time a great number of observations. See my experiments on the irritation of the muscular and nervous fibre, (in German) vol. i. p. 8; my letter to M. de Fourcroy, dated 5 Fluviose an 6 ; my Tableau géologique de l'Amérique Méridionale (Journal de Physique 1800); and my Géographie des Plantes, p. 117. The direction of high chains of mountains appears to have the greatest influence on the direction of the beds, even at consider. able distances from the central crest. This influence is manifest in the Pyrenees, Mexico, and especially in the Upper Alps. See the judicious observations which M. Ebel, a learned mineralogist has published on this subject under the title of, On the Construction of the Chain of the Alps (in German) vol. i. p. 220; vol. ij.
201 - 215. & p.357. * From 984 to 13}4 feet. Trans.