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 minerals by means of amalgamation, from those obtained by smelting. A mare 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 produced in 38 years gold and silver to the value of 165 millions of piastres † and that from 1786 to 1803, the annual average produce has been 556,000 marcs of silver equal to 4,727,000 piastres. All the veins of Hungary and Transylvania together, only yield 85,000 marcs of silver §.

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 1o. de Enero 1766 husta 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,

364,911 lb. Troy. Trans.

$55,686 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 Guanuaxuato, is the clay slate (thon schiefer) which reposes on the granite rocks of Zacatecas and the Peñon Blanco.* It is of an ash-grey or greyish-black frequently intersected by an infinity of small quartz veins, which frequently pass into talk-state (talk schiefer) and into schistous chlorite. I consider this clay slate as a 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 † at considerable depths. On digging the great pit (tiro general) of Valenciana, they discovered banks of syenite of Hornblend slate (Hornblend schiefer) and true serpentine, alternating with one another, and forming subordinate beds, in the clay slate. This extraordinary phenomenon of a syenite alternating with the 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 mine of Valenciana.

In the mines of Mellado, Animas and Rayas.

2624 feet. Trans.

Or from South-East to North-West. I have been struck ever since 1791, with this great law of the parallelism of the beds, which are discovered in immense extents of country, and which may be regarded as one of the most curious pheno

degrees to the south west. This is the direction of the greatest part of the very old rocks of Mexico.

Two yery different formations repose on the clay slate the one of porphyry at considerable elevations to the east of the valley of Marfil, and to the north west of Valenciana; and the other, of old freestone 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; and 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 Pluviose an 6; my Tableau Geologique de l'Amerique Meridionale (Journal de Physique 1800;) and my Geographie 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 considerable 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. 220; vol. ii. p. 201-215 & p. 357.

p.

*From 984 to 1314 feet. Trans.

surrounding plains.

In the country they go

by the name of buffa. Enormous balls with concentrical beds, repose on insulated rocks. These porphyries give a savage character to the environs of Guanaxuato, calculated to astonish the European traveller, who imagines that nature never deposits great metallick wealth but in mountains with round tops, and in places where the surface has a gentle and uniform undulation. This porphyry of which the Sierra de Santa Rosa is chiefly composed, is generally of a greenish colour; but it varies very much according to the nature of its base; and the chrystals which it contains. The oldest beds appear to be those of which the base is hornstone* (hornstein) or compact felspar. The most recent on the other hand, contain vitreous felspar, inchased in a mass, which sometimes passes into the petrosilex jadien, and sometimes into the pholonite or klingstein of Werner. The

Being a scholar of Werner, and of the school of Freiberg, I every where name in my works Hornstein a mineral which forms transitions into quartz, calcedony, and feuerstein (pyromaque). The hornsteine of the German mineralogists are, the Quartz-agathes, grossier et xyloides of M. Hauy, the neopetres of Saussure, and the siler cornés of M. Brogniart. This note appeared to me indispensable, on account of the confused synonomy of the denominations protosilex, pierre de corne, and roche de corne.