sumption of salt would be very inconsiderable in Mexico, because the Indians, who constitute a great part of the population, have never abandoned the old custom of seasoning their food with chile or pimento instead of salt.

NOTE A.

In speaking of the common metals which may be found in Mexico, I think the probability of their value, as an object of research, is overrated. Not that I doubt that they may be found, and perhaps in some instances abundantly; but unless fuel could be supplied in much larger quantity than at present, it cannot pay to work them.-For instance, iron and copper require such a great weight of coal that they cannot be worked to advantage without it; and when we take into account the expense of transporting and erecting machinery, the fuel required for merely draining the mines, the rate of wages of the miners of Mexico, and the charge of carrying the produce to the coast, we shall see at once, that though all these expenses may be borne by silver and gold mines, nothing but a great falling off in the supply from other countries, and a proportionable advance in price, can warrant the application of them to mines of the inferior metals.

An exception may be made in respect to mercury and lead.— The former may indeed be reckoned as a precious metal, and one of which the supply is limited, being only now found in Spain and Germany: it is also, according to the present practice, essential in Mexico for the extraction of gold and silver from the ores: and though I am of opinion that, if fuel can be obtained, it would be better to discontinue the use of mercury, yet I would strongly recommend M. de Humboldt's remarks to the attention of persons working mines in Mexico, and that

every inquiry and research should be made as to veins likely to produce quicksilver.

If smelting should ever be extensively practised in Mexico instead of amalgamation, lead would be the substitute for quicksilver, and a great consumption of it would take place, as in the process much would be dissipated or destroyed. This demand for it on the spot would render it valuable, and it would be applied in the state of ore. There is also every reason to think that the lead ores of Mexico may frequently contain silver in sufficient proportion to make it worth while to work them. J. T.

NOTE B.

Coal.-M. de Humboldt mentions this substance in the preceding chapter as having hitherto been found only in New Mexico, which is far too distant north for the supply of any of the mines now likely to be worked. It has however been also found in Peru and Chili, and some late accounts warrant the belief that it may be yet discovered within reach of the silver mines.

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In a letter which I had the honour to receive, containing some information from M. de Humboldt himself, dated at Paris, on the 12th Jan. last, (1824,) he tells me, that " he thinks that he "has insisted too much in his works upon the difficulties of "supplying fuel for steam engines: That this difficulty is not so great in general in Mexico as in Peru, as far as relates to "wood; because, in the latter, nature has deposited the mineral "treasures at such great elevations, that they are found almost " at the highest limits of all vegetation; whereas, upon the ta"ble-land of Mexico, though wood is not plentiful, yet a good "deal is to be found, as at Guanaxuato, from its proximity to "the Sierra de Santa Rosa: That there are also good woods near Real del Monte, at l' Oyamel, and at Cerro del Sacal." He also says, "that some coal has been found, but that he " does not know of any in the domains of Count Regla,” which had been reported to be the case; and he goes on to observe, "that the formations of basalt and amygdaloid, which are com"mon there, might indicate the presence of Lignite or Bovey coal, which would be very useful.”

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This observation of M. de Humboldt, has, in some degree, been confirmed, as there have lately arrived in England some specimens of a kind of coal from the district he speaks of, which, though not exactly the same as the one he mentions, is nearly allied to it, and appears to be what Professor Jameson has called pitch coal. It has the appearance of jet, is found in several coal-fields in Scotland, in some in England, and in Hungary, where the minerals generally remarkably resemble those of Mexico.

The letter above mentioned goes on to say, "that he had

seen secondary formations resting upon the metalliferous por"phyries about Actopan and Totonilcos; so that, geognostically "speaking, it is possible that true coal might be discovered "there." Some further remarks on this subject will be found in the next chapter.-J. T.

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CHAPTER VIII.

Geological description of the mining districts of Mexico. WHEN We examine the solid mass of our globe, we soon perceive that some of the substances, which descriptive mineralogy has made known to us separately, are found in constant associations, and that those associations, which are called compound rocks, do not vary, like organized beings, according to the difference of latitude, or of the temperature under which they are placed. Geognosts, who have travelled through the most distant countries, have not only found, for the most part, in the two hemispheres, the same simple substances, quartz, feldspar, mica, garnet, and hornblende; but they have also observed that mountainmasses display every where the same rocks; that is, the same assemblages of mica, quartz, and feldspar, in granite; of mica, quartz, and garnets, in micaslate; and of feldspar and hornblende, in syenite. If it has sometimes been considered, that a rock belongs exclusively to a single portion of the globe, subsequent researches have shown, that it also occurs in regions the most distant from its first locality. Thus we are almost led to admit, that the formation of rocks has been independent of

GEOLOGY OF MINING DISTRICTS.

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the diversity of climates, and perhaps anterior to its existence.* There is an identity even in those rocks where organized bodies are the most variously modified.

Transition clay-slatet forms far greater masses in the globe than primitive clay-slate. The latter is generally subordinate to mica-slate; as an independent formation, it is as rare in the Pyrenees and the Alps, as in the Cordilleras. In South America, between the parallels of 10° north and 7° south, I saw transition clay-slate only on the southern declivity of the littoral chain of Venezuela, at the entrance of the Llanos of Calaboza. The basin of the Llanos, the bottom of an ancient lake covered with secondary formations (red sandstone, zechstein, and clay-gypsum), is bounded by a band of intermediary formation of clay-slate, black limestone, and euphotide, connected with transition greenstone. Gneiss and mica-slate, between the valleys of Avague and the Villa de Ceura, constitute only one formation on which clay-slate reposes in conformable position, in the ravines of Malpasso and Piedras azules (direction N. 52° E.; inclin. 70° towards the N.W.), of which the lower

* Humboldt, Geography of Plants, 1807, p. 115. Idem, Views of the Cordilleras, vol. i. p. 122.

†The transition clay-slate in Cornwall is called killas, and indeed the same general term includes primitive clay-slate. This remark will enable the miners of that county to understand the description of rocks in the text.-J T.