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ble has its value only inasmuch as we connect the rock we wish to make known, to those by which it is immediately succeeded above and below. Mineralogical facts alone may be presented singly; positive geognosy is a science occupied with the relation and connexion of facts; and in describing any one portion of the globe, we ought not to limit our view, and stop at the study of a particular bed.

Central table-land, valley of Mexico; tract between Pachuca, Moran, and La Puebla. An enormous mass of transition porphyry rises to the mean height of 1200 to 1400 fathoms above the level of the sea. It is covered in the valley of Mexico, and at the south towards Cuernavaca and Guchilaque, with basaltic and cellular amygdaloid (tetzontli in Mexican); and towards the east and north-east (between Tlascala and Totonilco) with secondary formations. The porphyry, probably hid at first beneath the mountain limestone of Mescala and then in the Llanos of San Gabriel (near the bridge of Istla) beneath trachytic conglomerates and a po rous amygdaloid, is identical with that which reappears, 45 miles further north, and 800 fathoms higher, on the banks of the lake of Tezcuco. In the fine valley of Mexico the porphyritic rock pierces the cellular amygdaloid in the hills of Chapoltepec, of Notre Dame de la Guadeloupe, and of Peñol de los Baños. It exhibits several very remarkable varieties: 1. reddish-gray, a little argillaceous, with

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out distinct stratification, containing crystals of hornblende, and common feldspar in equal parts (level dug in the rock of Chapoltepec); 2. black or darkish-gray (sometimes fissile and spongy) stratified by beds from 3 to 4 inches thick, with a basis of compact feldspar, fracture dull, smooth, or imperfectly conchoidal, resembling more the fracture of lydian stone than that of pitchstone, containing small crystals of glassy feldspar and olive-green pyroxene, almost destitute of hornblende, and often covered at their surface by superb masses of reniform hyalite, or Muller's glass (Peñol de los Baños; dir. N. 60° W.; incl. 60° N. E.); 3. red, earthy, with a quantity of large crystals of common decomposed feldspar (salt-works of the lake of Tezcuco, where the Peñol is covered by ancient Aztic sculptures). The porphyry of the valley of Mexico furnishes not only springs of pure water, which is conveyed to the town by long and magnificent aqueducts, but also acidulated thermal waters, some warm, and others cold. Here are also found (and this is a very remarkable fact) naphtha and petroleum (promontory of the sanctuary of Guadeloupe), as in the primitive mica-slate of the vicinity of Araya and Cumana. Although this porphyry appears below the porous amygdaloid, and is seen (Cerro de las Cruces and Tiangillo, Cuesta de Varientos and Capulalpan, Cerro Ventoso, and Rio Frio) in all the circular outline of the basin of Tenochtitlan, (the bottom of an ancient lake partly dried,) it is only towards

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the north-east (Pachuca, Real del Monte, and Moran) that it has been found to be argentiferous.

Several rich veins traverse a mass of porphyry above 700 feet in thickness, from the mine of San Pedro, at the summit of Cerro Ventoso (1461 fathoms), as far as the bottom of the ancient wells of Encino (1170 fathoms), in the Real de Pachuca. This rock, which would have formerly been called petrosilex, or hornstone-porphyry, is generally greenish-gray, sometimes prase-green, with a scaly fracture, and giving fragments with sharp edges. The paste is probably a compact feldspar, having a large proportion of silica; and contains, not quartz and mica, but crystals of common feldspar and hornblende. In general, the latter substance is not very abundant, and when the porphyry is argillaceous, or merely earthy, we recognise the hornblende only by spots with striated surfaces, and of a very dark green. The beds which are argillaceous and softer (clay porphyry of Moran) appear to be below the harder and more tenacious beds. Strata of phonolite (klingstein) are found subordinate to both; it is smoke-gray, or leek-green, divided into tables or plates that are very sonorous. It is not, however, altogether a porphyry-slate of the trachyte formation; for the phonolitic mass does not contain thin crystals of glassy feldspar, but crystals of grayish-white common feldspar, constantly accompanied with a little hornblende. All these argentiferous porphyries of Moran and Real del Monte are very re

gularly stratified (general direction as in the valley of Mexico, N. 60° W., incl. 50°—60° at N, E.): they are irregularly columnar only in the Organos of Actopan (Cerro de Mamanchota, summit 1527 fathoms) and the Monjas of Totonilco el Chico; if indeed the rock of Organos, the mass of which is 3000 feet thick, reckoning only the porphyries visible above the neighbouring plains, is identical with the rock of Moran. The latter contains fewer crystals of hornblende; neither of these rocks is fissile or porous, and it is at the foot of the grotesque peaks of Monjas that the rich veins of Tononilco el Chico are found. The argentiferous porphyries of Pachuca and Moran which I have just described, present no character that should separate them from the transition formation: they are even covered, between the baths of Totonilco el Grande and the cavern of Madre de Dios, or the pierced rock, by enormous calcareous masses, and by sandstone and gypsum.

Group of porphyries of Guanaxuato. It is this group that determines most clearly the relative age, or, to express myself with more precision, the maximum of the antiquity of the Mexican porphyries; if indeed those of which we have just indicated the positions are of the the porphyries of Guanaxuato. of those porphyries on rocks mediary formation is evident. Noria, and in the Cañada de olive-green porphyry, containing glassy feldspar in

same formation as The superposition belonging to interNear the farm of Queretaro, a slaty

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microscopic crystals, is superposed on a transition clay-slate with lydian stone. This superposition is equally certain near Guanaxuato, and particularly near Santa Rosa de la Sierra. The porphyries of this district have in general a concordant position (a parallel direction and inclination) with the strata of clay-slate. They are eminently metalliferous; and the famous vein of Guanaxuato (Veta Madre), making the same angle with the meridian as the veins of Zacatecas, Tasco and Moran (N. 50° W.); has been worked successively on a length of 12,000 fathoms, and a thickness of 20 to 25 fathoms. It has furnished, in 230 years, more than 39,000,000, and traverses both porphyry and transition-slate. The first of these rocks forms, at the east of Guanaxuato, gigantic masses, that have at a distance the most singular aspect, resembling walls and bastions. These perpendicular ridges, rising more than 200 fathoms above the surrounding plains, bear the name of buffas; they are destitute of metals, appear to have been heaved up by elastic fluids; and are regarded by the Mexican miners, who see them placed also at Zacatecas on a very metalliferous transition clay-slate, as a natural indication of the riches of those countries. When we consider the porphyries of La Buffa de Guanaxuato, and those of the formerly celebrated mines of Belgrado de San Bruno, la Sierra de Santa Rosa, and Villalpando, in the same point of view, we think we perceive in their latest strata,