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we should not find angular fragments of its roof contained in its mass, as we generally observe on points where the roof is a slate charged with carbone, and the wall a talc slate. In a vein, the roof and the wall are deemed anterior to the formation of the crevice, and to the minerals which have successfully filled it; but a bed has undoubtedly pre-existed to the strata of the rock which compose its roof. Hence we may discover in a bed fragments of the wall, but never pieces detached from the roof.

The veta madre of Guanaxuato, exhibits the extraordinary example of * a crevice formed according to the direction and inclination of the -strata of the rock. Towards the south east from the ravin of Serena, or from the mines of Belgrado and San Bruno, which are very fully wrought, to beyond the mines of Marisanchez, it runs through porphyritic mountains; and towards the north east on leaving the pits of Guanáxuato, to the Cerro de Buena Vista, and the Cañada de la Virgen, it traverses the clay slate (thon scheifer). Its extent varies like that of all the veins of Europe. When not ramified it is generally from 12 to 15 metres* in breadth; sometimes it is even strangledt to the extent of half a metret ; and it is for the most part found divided into three masses, (cuerpos) separated either by banks of rock, (caballos) or by parts of the gangue almost destitute of metals. In the mine of Valenciana the veta madre has been found without ramification, and of the breadth of 7 metress, from the surface of the ground to the depth of 170 metres|l. At this point it divides into three branches, and its extent, reckoning from the wall to the roof of the entire mass, is 50 and sometimes even 60 metres q. Of these three branches of vein, there is in general but one alone which is rich in metals; and sometimes when all the three join and dray one another, as at Valenciana near the pit of San Antonio, at a depth of 300 metres**, the vein contains immense riches on an extent (puissance) of more than 25 metrestt. In the pertinencia de Santa Leocadia, four branches are observable.

oun.

* M. Werner in the Theory of Veins, $ 2. expressly says, “ that the depositories of minerals almost always cut the “ banks of the rock.” This great mineralogist seems to have intended to indicate by these words, that there may be true veins parallel to the folia of a clay, or micaccorts slate.

* From 38 to 48 feet. Trans.

+ At the place of assemblage of the pit of Santo Christo de Burgos, in the Mine of Valenciana.

19 inches. Trans. $ 22 feet. Trans. 1 557 feet. Trans. I 164 and 196 feet. Trans. ** 984 feet. Trans. ft 81 feet. Trans.

A trum of which the inclination is 65° separates from the inferior branch, (cuerpo baxo) and cuts the folia of the rock of the wall. This phenomenon, and the great number of druses, abounding with amethyst chrystals, to be found in the mines of Rayas, which affect the most different directions, are sufficient to prove that the veta madre is a vein, and not a bed. Other proofs not less convincing might be drawn from the existence of a vein, (veta del caliche) wrought in the compact limestone of Animas, which is parallel to the principal vein of Guanaxuato, and has exhibited the same silver minerals. Is this identity of formation ever found between two metalliferous beds, which belong to rocks of very different antiquity ?

The small ravins into which the valley of Marfil is divided, appear to have a decided influence on the richness of the veta madre of Guanaxuato, which has yielded the most metal where the direction of ravins, and the slope of the mountains, (flaqueza del Cerro) have been parallel to the direction and inclination of the vein. When we stand on the elevation of Mel. lado, near the pit which was dug in 1558, we observe that the veta madre is in general most abundant in minerals towards the north west, towards the mines of Cata and Valenciana ; and that to the south east towards Rayas and Santa Anita, the produce has been at once richer, rarer, and more inconstant. Besides in this celebrated vein, there is a certain middle region which may be considered as a depositary of great riches; for above and below this region, the minerals have contained an inconsiderable share of silver. At Valenciana the rich minerals have been in the greatest abundance, between 100 and 340 metres* in depth below the mouth of the galery, This abundance appeared at Rayas at the surface of the earth; but the galery of Valenciana is pierced according to my measurementst, in a plain which is more than 156 metrest above the level (galerie d'ecoulement) of Rayas; which might lead us to believe that the depository of the great wealth of Guanaxuato is found in this part of the vein, between 2130 and 1890 metres of absolute height above the level of the oceang. The deepest works of the mine of Rayas, (los planes) have never yet reached the inferior limit of this middle region; while the bottom (das tiefste) of the mine of Valenciana, the galery of San Bernardo has unfortunately passed this limit more than 70 metresll. Hence

* Between 328 and 1115 feet. Trans.

+ See my Recueil d'Obserpations Astronomiques, Vol. į p. 324. No. 332—357. .

$ 511 feet. Trans. : Between 6987 and 6199 feet. Trans.

| 229 feet. Trans.

the mine of Rayas continues to furnish extremely rich minerals, while at Valenciana they have endeavoured for some years, to supply by the extraction of a greater quantity of minerals, the deficiency in their intrinsic value.

The mineral substances which constitute the mass of the vein of Guanaxuato, are common quartz, amethyst, carbonate of lime, pearl spar, splintery hornstone, sulfuretted silver, ramular native silver, prismatic black silver, deep red silver, native gold, argentiferous galena, brown blende, spar iron, and pyrites of copper and iron. We observe besides though much more rarely, crystalized felspar (the rhomboidal quartz of the Mexican mineralogists) calcedony, small masses of spar-fluor, capillary quartz (haarför. miger quartz), grey copper ore (fahlerz) and bacillary carbonated lead. The absence of the sulfate of barytes and muriated silver, distinguishes the formation of the vein of silver from that of Sombrerete, Catorce, Fresnillo, and Zacatecas. Those mineralogists who are interested in the study of regular forms, find a great variety of crystals in the mines of Guanaxuato, and especially in the mines of red and black sulfuretted silver, and in the calcareous spars, and the brown spar.*

* On the pearled spar of Guanaxuato, see Klaproth's Beiträge, B. iv. p. 128. This variety of brown-spar (braunspath) exhibits microscopic crystals embricked and collected

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