passages to rocks which in Europe are generally placed among trachytes.

In the vicinity of Guanaxuato, porphyries predominate, which have a paste of compact feldspar, are gray and olive-green, and contain imbedded lamellar feldspar (not glassy), either in crystals almost microscopic (Buffa) or in very large crystals (mines of San Bruno and Tesoro). Decomposed hornblende, which probably tinges with green the whole mass of these rocks, is only distinguished by irregular spots. In ascending towards the Sierra (Puerto de Santa Rosa, Puerto de Varientos), the porphyry is often composed of balls with concentric layers; its paste becomes darkish-gray, semi-vitreous (pitchstone-porphyry), and contains a little crystallized mica and grains of quartz. The auriferous veins near Villalpando traverse a green porphyry with a base of phonolite, in which we perceive only some small and thin crystals of glassy feldspar. It is difficult to distinguish this rock from trachytic porphyry-slate; I have seen it covered with an earthy yellowish-white porphyry (mine of Santa Cruz), and with an ancient conglomerate (mouth of the mine of Villalpando), which evidently represents the red sandstone, and the lower beds of which pass to grauwacke.

The porphyries of the equinoctial region of Mexico contain, although rarely, besides some disseminated garnets (Izmiquilpan and Xaschi), sulphuret of mercury (San Juan de la Chica; Cerro

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del Fraile, near the Villa of San Felipe; Gasave, at the northern extremity of the valley of Mexico), tin (El Robedal, and la Mesa de los Hernandez), and alumstone (Real del Monte, according to M. Sonneschmidt). This latter substance seems to give to the porphyritic rocks a nearer affinity to real trachytes; although in South America (peninsula of Araya, Cerro del Distiladero and Chupariparu) I saw a clay-slate which belonged rather to the primitive than to the intermediary formation, traversed by veins, I will not say, of alumstone (alumstein), but of native alum, of which the Indians sell pieces of the size of an inch at the market of Cumana. The cinnabar of the porphyries of San Juan de la Chica, the argillaceous beds of Durasno containing both coal and cinnabar and placed on a porphyry with much hornblende, are phenomena well worthy of attention. The geognosts who,

like myself, attach more importance to the position than to the mineralogical composition of rocks, will, no doubt, connect the porphyries and clays of Davasno with the deposites of mercury which the formation of the red sandstone and porphyry exhibits in both hemispheres (duchy de Deuxponts and Cuença, between Quito and Loxa). The last beds of the transition formation are found every where in close connexion with the most ancient beds of the secondary formation.

The celebrated silver vein of Bolaños contains its greatest riches in an amygdaloid interposed in

porphyry. In Hungary, England, Scotland, and even in Germany, rocks of amygdaloid and porphyry belong to grauwacke, to clay-slate, to transition limestone, to red sandstone, and to coal sandstone. The metalliferous porphyry of Gua naxuato simply covers clay-slate, without forming in it subordinate beds; but a syenite analogous to that which occurs in the mine of Valenciana, in the midst of intermediary clay-slate,. alternates thousands of times, on a surface of more than twenty square leagues, with transition greenstone, between the mine of the Esperanza and the village of Comangillas. In that region the syenitic rock is destitute of metals, but at Comanja it is argentiferous, as it is also in Saxony and in Hungary.

I had the opportunity of observing the formation of red sandstone in the equinoctial region of the New Continent, north and south of the equator, in six different places; in New Spain (from 1100 to 1300 fathoms high), in the steppes or Llanos of Venezuela (30-50 fathoms), in New Granada (50-1800 fathoms), on the southern table-land of the province of Quito (1350-1600 fathoms), and in the western valley of the Amazon (200 fathoms).

The schists and transition porphyries of Guanaxuato (table-land of Anahuac), of which we have given a detailed description above, are covered with a formation of red sandstone. This formation fills the plains of Celaya, Salamanca, and Burras (900 fathoms); it there supports

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limestone very analogous to that of Jura, and lamellar gypsum. It extends by Cañada de Marfil to the mountains that surround the town of Guanaxuato, and appears in insulated spots in the Sierra de Santa Rosa, near Villalpando (1330 fathoms). This Mexican sandstone has the most striking resemblance with the rothe todte liegende of Mansfeld in Saxony. It contains angular fragments of lydian stone, syenite, porphyry, quartz, and flint (splittriger hornstein). The cement that unites these fragments is argillo-ferruginous, very tenacious, yellowish-brown, and often (near the river of Serena) of a brick-red colour. Beds of coarse conglomerate, containing fragments two or three inches in diameter, alternate with a finegrained conglomerate, sometimes even (Cuevas) with a sandstone consisting uniformly of grains of quartz. Coarse conglomerates abound more in plains and ravines than on the heights. I thought I perceived in the most ancient beds (mine of Rayas) a passage from the red sandstone to grauwacke; the pieces of imbedded syenite and porphyry become very small; their outlines are indistinct and appear as if softened into the mass. This conglomerate (frijolillo de Rayas) must not be confounded with that of the mine of Animas, which is whitish-gray, and contains fragments of compact limestone. In the red sandstone of Guanaxuato, as well as in that of Eisleben in Saxony, the cement is often so abundant (road from Guanaxuato to

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Rayas and Salgado) that the imbedded fragments are no longer distinguished. Argillaceous beds, from three to four inches thick, then alternate with the coarse conglomerate. The great formation of red sandstone, superposed on metalliferous clayslate, appears in general only when supported by transition porphyry (Belgrade, Buffa de Guanaxuato); but we see it distinctly placed on the latter rock at Villalpando. I found no petrified shells, nor any trace of coal or fossil wood in the red sandstone of Guanaxuato. These combustible substances occur frequently in other parts of New Spain, especially in those which are least elevated above the level of the sea. In the interior of New Mexico, coal is known not far from the banks of the Rio del Norte; other deposites of it probably are hid in the plains of Nuevo Sant-Ander and the Texas. At the north of Natchitoches, near the coal-mine of Chica, subterraneous detonations are heard from time to time, from an insulated hill, occasioned perhaps by the inflammation of hydrogen gas mixed with atmospheric air. Fossil wood is common in the red sandstone that extends towards the north-east of the town of Mexico. It is also found in the immense plains of the intendance of San Luis Potosi, and near the town of Altamira. The coal of Durasno (between TierraNueva and San Luis de la Paz) is placed below a bed of clay containing fossil wood, and over a bed of sulphuret of mercury which covers the porphyry.