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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.

CHAPTER IX.

Geological constitution of mines-position and extent of mineral veins-elevation of mines-comparative wealth of mines-distance between principal mines-nature of ores—mean riches of ores.

As the subject of the disposition of the mineral treasures of Mexico in the bosom of the earththe direction, inclination, and character of the veins, is of primary importance, we shall communicate the result of some further observations on those interesting phenomena, together with some details as to the comparative wealth of the most celebrated veins.

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. These difficulties are increased when it happens, as in the mountains of Mexico, that the veins, 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 Mexico, or which have been wrought within the two last centuries, we

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should undoubtedly perceive, in the materials and structure of these veins, analogies indicative of a simultaneous origin; we should find that the matters comprising the veins 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 1200 miles, 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 or lodes are the object of the most considerable operations; and the ores disposed in beds or in masses are not frequent. The Mexican lodes are for the most part found in primitive and transition rocks, and rarely in the rocks of secondary formation.

On taking a general view of the metalliferous repositories, we find that the Cordilleras of Mexico contain veins in a great variety of rocks, and that those rocks, which at present furnish almost the whole silver annually exported from Vera Cruz, are,

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