* Recent localic modification 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 disposition 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, confined 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 scat- In the present state of the country, the veins. VOL. III. incun, stance, here, dates, the cowinnich ment of the puriferious modification a Rubriquent to the manual labour 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-tin) 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 Guanaxuato, is a porphyry, of which the basis is somewhat akin to klingstein (phonolite), and in which amphibole is extremely rare. Several 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. 1 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 |