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IX. Intendancy of Oaxaca. . From the 16° 35' to the 17° 55' of north latitude, and from the 98° 15' to the 100° ( of west longitude. Diputaciones de Mineria, or Districts.
37. Oaxaca. Reales, or Places surrounded by Mines : Zologa ; Talea ; Hueplotitlan; La Aurora de Ixtepexi ; Villalta ; Ixtlan ; Betolatia ; Huitepeque; Rio de San Antonio ; Totomistla; San Pedro Nesicho; Santa Catalina; Lachateo;. San Miguel Amatlan ; Santa Maria Iavecia ; San Mateo Capulalpa ; San Miguel de las Peras.
X. Intendancy of Puebla. From the 18° 15, to the 20° 25' of north latitude, and from the 99° 45' to the 100° 50' of west longitude.
Scattered Mines : La Canada ; Tulincingo; San Miguel Tenango; Zautla ; Barrancas; Alatlanquetepec; Temetzla ; Ixtacmaztitlan.
XI. Intendancy of Vera Cruz. From the 20° 0' to the 21° 15' of north latitude, and from the 99° 0 to the 101° 5' of west longitude.
Scattered Mines : Zomelahuacan; Giliapa; San Antonio de Xacala.
XII. Old California. - Mine. Real de Santa Ana.
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. The naturalist may distinguish the relative antiquity of the different formations, and he is enabled to discover laws in the stratification of rocks, in the identity of beds, and often even in the angles which they form, either with the horizon or the meridian of the place; but how can he recognize the laws which have determined the dis"position of the metals in the bosom of the earth, the extent, the direction, and inclination of the veins, the nature of their mass, and their particular structure? How can he draw general results from the observation of a multitude of small phenomena, modified by causes of a purely local nature, and appearing to be the effects of an action of chemical affinities, circumscribed to a very narrow space? 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 formation. If we, possessed an accurate description of the four or five thousand veins actually wrought in New Spain, or which have been wrought within the two last centuries, we should undoubtedly perceive in the mass and structure of these veins, analogies indicative of a simultaneous origin; we should find that these masses (gangausfüllungen) 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 400 leagues, I shall not venture to sketch a general view of the Mexican mines, considered under their geological relations. I shall content myself merely with indicating the rocks, which yield the greatest part of the wealth of New Spain.
In the actual state of the country, the veins are the object of the most considerable operations; and the minerals disposed in beds or in masses are very rare. The Mexican veins are to be found for the most part in primitive and
transition rocks (ur-und übergangs-gebürge), and rarely in the rocks of secondary formation which only occupy a vast extent of ground to the north of the Tropic of Cancer, to the east of the Rio del Norte, in the basin of the Mississippi, and to the west of New Mexico, in the plains watered by the rivers of Zaguananas and San Buenaventura which abound in muriatic salts. In the old continent granite, gneiss and micaceous slate (glimmer-schiefer)
constitute the crest of high chains of mountains, · But these rocks seldom appear outwardly on the ridge of the Cordilleras of America, par- : ticularly in the central part contained between the 18° and 22° of north latitude. Beds of amphibolic porphyry, greenstone, amygdaloid, basalt and other trap formations of an enormous thickness cover the granite and conceal it from the geologist. The coast of Acapulco is formed of granite rock. Ascending towards the table land of Mexico we see the granite pierce through the porphyry for the last time between Zumpango and Sopilote. Farther to the east in the province of Oaxaca the granite and gneiss are visible in table lands of considerable extent traversed by veins of gold.
Tin which after Titanium, Scheelin and Molybdena is the oldest metal of the globe, has never yet as far as I know been observed in the granites of Mexico; for the fibrous tin (nood-tin) of the
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 Con manja, a syenite apparently of antient formation contains a seam of silver; the mine. of Guanaxuato the richest of all America crosses à primitive slate (thon-schiefer) which frequently passes into the talk-slate (talk-schiefer): 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 géology 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 vitre ous 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