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Does it belong to very recent lignites? or, ought we not rather to admit that these combustible substances of Durasno, these clays and semi-vitreous porphyries (pechstein porphyre), globular and covered with mammillated hyalite, porphyries which in other parts of Mexico (San Juan de la Chica, Cerro del Fraile near the Villa of San Felipe) contain deposites of sulphuret of mercury, are connected with the great formation of red sandstone? There is no doubt that this formation is as rich in mercury in the new continent as in the west of Germany; it is found there when the porphyries are wanting (Cuença, table-land of Quito); and if the union of veins of tin with veins of cinnabar, in the porphyries of San Felipe, appear at first to remove porphyritic rocks which abound in mercury from those of the red sandstone, we must recollect that transition clay-slate and porphyries (Hollgrund near Steben, Hartenstein) are also sometimes stanniferous in Europe.

I place in the suite of coal-sandstones of Guanaxuato, a formation that is somewhat doubtful, which I have already described, in my Political Essay on New Spain, by the name of lozero, or feldspathic conglomerate; it is an arenaceous rock, reddish-white, and sometimes apple-green, which divides, like sandstone (leuben or waldplattenstein of Suhl), in very thin plates (lozas); it contains grains of quartz, small fragments of clayslate, and a quantity of crystals of feldspar partly

broken and partly entire. These various substances are connected together in the lozero of Mexico, as in the rock of porphyritic aspect of Suhl, by an argillo-ferruginous cement (Cañada de Serena, and almost the whole mountain of that name). It is probable that the destruction of the porphyry has had great influence on the formation of the feldspathic sandstone of Guanaxuato. The most experienced mineralogist might at first be led to take it for a porphyry with an argillaceous base, or for a porphyritic breccia. Around Valenciana the lozero forms masses of 200 fathoms in thickness, and which exceed in elevation the mountains formed by the intermediary porphyry. Near to Villalpando a feldspathic conglomerate, with very small grains, alternates by beds one or two feet thick, twenty-eight times with slate-clay of a darkish brown. I saw every where this conglomerate or lozero reposing on the red sandstone, and at the south-west declivity of Cerro de Serena, in descending towards the mine of Rayas, it appeared to me sufficiently evident that the lozero forms a bed in the coarse conglomerate of Marfil. I doubt, consequently, if this remarkable formation can belong to trachytic pumice-conglomerates, as M. Beudant seems to think from its analogy with some rocks in Hungary. The argillaceous cement becomes often so abundant that the imbedded parts are scarcely visible, and the mass passes to compact clay-stone (thonstein). In this

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state the lozero furnishes the fine building-stone of Queretaro, (quarries of Caretas and Guimilpa,) which is so much esteemed for architecture. I have seen columns fourteen feet high, and two feet and a half in diameter, flesh-red, brick-red, or peach colour. When in contact with the atmosphere those fine colours change to gray, probably by the action of the air on the dendritic manganese contained in the fissures of the rock. The columns of Queretaro have a smooth fracture, like that of the lithographic Jura limestone. With difficulty we discover in these clay-stones (argilolites) some. very small fragments of clay-slate, quartz, feldspar, and mica. I will not decide, if the unbroken crystals of lozero or feldspathic sandstone are developed in the mass itself, or are found there accidentally. I shall here confine myself to the observation, that in Europe this red sandstone and these porphyries are also sometimes characterized by a local suppression of crystals and imbedded fragments. The lozero appears to me to be a formation of superposed sandstone, perhaps even subordinate to the red sandstone; and if no rock entirely similar is found in the ancient continent, we see, at least, the first germ of this kind of pseudo-porphyritic structure in the layers of sandstone with feldspar crystals, broken or entire, which are sometimes imbedded in the great formation of red sandstone of Mansfeld and Thuringerwald.

NOTE 1.

An attentive perusal of the preceding observations on the geology of those parts of Mexico which contain the richest mines, will present striking resemblances, in some points, to what is observed in England. It is only within a few years, that the coincidence between the position of the most productive mines of Cornwall, with the junctions of different rocks has been much noticed. My own attention was first called to it by the communication of observations made in different parts of Europe by my friend Mr. Greenough, and I believe I was afterwards instrumental in calling the attention of our practical miners to the circumstance. The truth of the remark is now universally admitted in that district, where it will appear that our great mines are either near the junction of the killas (clay-slate) - with the granite, or they are in killas which is intersected by channels of elvan (inclined beds of porphyry). So prevalent is the opinion now become, that some of the most intelligent miners would hesitate to recommend trials which involve much expense, on lodes or veins discovered where the different rocks are not thus associated, or in what has in Cornwall been significantly called a clean country.

Our lead mining districts in England are also to be found near junctions, either where the coal measures rest on the limestone, as in North Wales, or where rocks are interstratified, as in Derbyshire, Yorkshire, or Cumberland and Durham.

We see this theory confirmed in Mexico: in the succeeding chapter we shall find that at Guanaxuato the richest veins in the world are in killas, containing beds of syenite, and in porphyries, which we should call elvan; and that the best mines on this extraordinary lode are near the junction of the rocks.

At Sombrerete the lode is in limestone containing lydian stone, a kind of flinty slate nearly allied to chert, which in England is found with the limestone near some of our best lead mines, and is esteemed a favourable circumstance, as it is also at Ecton mine for copper.

In Zacatecas and Catorce, where the mines have been extremely rich, we find them in intermediary formations of clay

slate, grauwacke and limestone. The veins of Tasco traverse both limestone and clay-slate.

Real del Monte, Moran and Pachuca, which are nearly together, and Bolaños which is very distant from them, exhibit porphyry as the prevalent rock: we observe, however, that it is stratified with beds which are of the argillaceous kind, and that strata of phonolite, which is a kind of basalt or whin stone, occur in them; and that amygdaloid, another rock of the same nature, forms junctions at no great distance.

We cannot too much value the industry and accuracy of the learned author, which have enabled us to compare facts so`important, at such a distance from our usual means of information. J. T.

NOTE 2.

The lozero of Guanaxuato, which M. Humboldt places in the suite of coal sandstones, will deserve particular attention on the part of those who may undertake the mines of that important district. It does not indeed appear that any coal has been discovered there; but as this rock seems to resemble some of the sandstones which occur among coal formations, and as fuel for steam-engines and smelting will be an article of the utmost importance, no effort should be spared for its discovery, and no exertion should be abandoned while any chance exists. J. T.