Page images
PDF
EPUB

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

HELE

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. 52o 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.

beds are green, steatitic, and mixed with 'hornblende, and the upper are of a greyish-green and darkish-blue colour. This clay-slate contains (like that of Steben, in Franconia, the duchy of Nassau, and of Peschels-Mühle in Saxony) beds of greenstone, sometimes massive, at other times of a globular structure. , · The famous vein of Guanaxuato in New Spain, which, from 1786 to 1803, has produced yearly, on an average, 343,000 lbs. troy of silver, also traverses transition clay-slate. This rock, in its lower strata, passes, in the mine of Valenciana, (at the height of 932 fathoms above the level of the sea,) to a talcose slate; and I described it in my Political Essay as placed on the limit of the primitive and intermediary formations. A more particular examination of the relations of position which I noted on the spot, and the comparison of the beds of syenite and serpentine which have been pierced in digging the great shaft, with the beds that are interposed in the transition formations of Saxony, of the Bochetta of Genoa, and of the Cotentin, convinced me that the 'clay-slate of Guanaxuato belongs to the most ancient intermediary forma. tions. We do not know whether its stratification be parallel and conformable with that of the granitegneiss of Zacatecas, and of Peñon blanco, which probably supports it, the contact of these formations not having been observed; but almost all the porphyry-rocks on the great table-land of Mexico

follow the general direction of the chain of mountains (N. 40°, 50° W.). This perfect concordance (gleichförmigkeit der lagerung) has been observed between the primitive gneiss and the transition clay-slate of Saxony (Friedricks-walde; valleys of Müglitz, Seidewitz, and Lockwitz); it proves that the formation of the intermediary series immediately succeeded that of the last beds of the primitive series. In the Pyrenees, as M. de Charpentier has observed, the former of these series is found in a different position (not parallel), sometimes in transgressive position (übergreifende lagerung) with the latter. I shall observe, on this occasion, that the parallelism between the stratification of two consecutive formations, or the absence of this parallelism, does not alone decide the question whether the two formations are united or not in the same primitive or secondary series ; it is rather the sum of these geognostic relations that solves that problem. The clay-slate of Guanaxuato is very regularly stratified (direct. N. 46° W.; inclin. 45° S.W.), and the form of the valleys has no influence on the direction or inclination of the strata. We distinguish there three varieties, which may be considered as three epochas of formation ; a clayslate of a silvery lustre, and steatitic, passing to a talc-slate (talk-schiefer); a greenish clay-slate, with a silky lustre, resembling chlorite slate ; finally, a black clay-slate, in very thin laminæ, surcharged with carbon, staining the figures like ampelite and the marly slate of zechstein, but not effervescing

sam

with acids. I have named these varieties in the order in which I observed them from below upwards, in the mine of Valenciana, which is 263 fathoms in perpendicular depth ; but in the mines of Mellado, Animas, and Rayas, the supercarburetted clay-slate (hoja de libro) occurs beneath the green and steatitic variety; and it is probable, that the strata which pass to talc-slate, chlorite, and ampelite, alternate several times with each other. . . The thickness of this formation of transition clay-slate, which I found at the mountain of SantaRosa near Los Joares, where the Indians collect ice in small artificial basins, is more than 3000 feet. It contains in subordinate beds, not only syenite (like the transition clay-slate of the Cotentin), but also, which is very remarkable, serpentine, and a hornblende-slate that is not greenstone. In digging in the massive rock, the great shaft of Valenciana, which has cost nearly 292,0001., the following strata were found, reckoning downwards for ninety-four fathoms of depth: an ancient conglomerate, representing the red sandstone; black transition clay-slate, strongly carburetted, in very thin laminæ; clay-slạte, blueish-grey, and containing magnesia ; hornblende-slate, greenish-black, a little mixed with quartz and pyrites, destitute of feldspar, not passing to greenstone, and altogether similar to the hornblende-slate (hornblend-schiefer) which forms beds in primitive gneiss and mica-slate; green serpentine of uneven fracture and fine grain, dull, but translucent on the edges, containing much

« ՆախորդըՇարունակել »