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 New Spain, 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 these vein materials (gang-ausfüllungen) 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 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 are the object of the most considerable operations; and the ores disposed in beds or in masses are not frequent. The Mexican veins are for the most part found 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 basket out on the ridge of the Cordilleras of America, particularly in the central part contained between the 18°, and 22° of north latitude. Strata 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 granitic rocks. 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 auriferous veins.

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 (wood-lin) of the

Gigante belongs to alluvial rocks, and the veins of tin of the Sierra de Guanaxuato are found in mountains of porphyry. In the mines of Comanja, a syenite, apparently of antient formation, contains an argentiferous vein. That of Guanaxuato, the richest of all America, traverses a primitive slate (thon-schiefer) which frequently passes into 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 ores 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 feldspar 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 feldspar in the porphyries of Spanish America. The rock which is intersected by the rich gold vein of Villalpando, near Guamaxuato, is a porphyry, of which the basis is somewhat akin to klingstein (phonolite), and

Several

in which amphibole is extremely rare. of these parts of New Spain bear a great analogy to the problematical rocks of Hungary, designated by M. de Born by the very vague denomination of saxum metalliferum. The veins of Zimapan, which are the most instructive in respect to the theory of the repositories of minerals, traverse porphyries of a greenstone base which appear to belong to trap rocks of a newer formation. These veins of Zimapan offer to oryctognostic collections a great variety of interesting minerals, such as fibrous zeolite, stilbite, grammatite, pycnite, native sulphur, fluor spar, barytes, suberiform asbestus, green garnets, 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 fire opal.

Among the transition rocks which contain ores of silver, we may mention the transitionlimestone (übergangs-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 are not veins, but masses of galena, of which some have yielded in a short space of time, according to the observation of M. Sonneschmidt, more than 124,000. quintals of lead.

The grauwacke alternating

with grauwacke-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 formations and transition rocks, but extend also to those of secondary formation. I know not whether the lead which is procuredin 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 (alpenkalkstein); and this rock rests on pudding-stone, with silicious cement, which may be considered as the most antient of secondary formations. The alpine limestone, and the jura limestone (jurakalkstein), 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. They are less productive in the strata of primitive slate (urthen-schiefer), which is as seen in the

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