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greenstone (diabase). These beds are in general from four to five decimeters * in thickness; and they are inclined by groups, sometimes to the north-east, sometimes to the west, and always at angles of from 50 to 60 degrees. In travelling from Valenciana to Ovexeras, we see several thousands of these banks of greenstone, alternating with a syenite, in which quartz is sometimes in greater abundance than felspar and hornblende. We find veins of greenstone in this syenite, and crevices filled with syenite in the beds of greenstone. This identity of the mass of the veins with the superimposed rocks, is a curious fact which seems to favour the tlieory of the origin of veins, laid down by Mr. Werner. t Near Chichimequillo, a columnar porphyry seems to rest on syenite. It is covered with basalt and basaltic breccia, from which the springs, of which the temperature is 96' 31 of the centrigrade thermometer, have their
It remains for me to give an account of two partial formations which occupy only a very small extent: a compact limestone (elcaliche) of a blackish grey color, belonging perhaps to
* From 15 to 19 inches. Trans. + Neue Theorie von der Entstehung der Gänge, 1791, p. 60.
205° of Fahrenheit. Trans,
transition rocks *, and a calcareous breccia (frijollilo). The latter, which I saw in the mine of Animas, at a depth of more than 150 metres f, is composed of round fragments of compact limestone, connected together by a calcareous cement. The clay slate of Valenciana serves for base to these two partial formations, one of which appears to owe its origin to the destruction of the other.
Such is, according to the observations made by me on the spot, the geological constitution of the country at Guanaxuato. The vein (veta madre) traverses clay slate as well as porphyry; and in both of these rocks it has proved very rich in ore. Its mean direction is h. 87 of the miner's compass f; and is nearly the same with that of the veta grande of Zacatecas, and of the veins of Tasco and Moran, which are all western veins (spathgänge). The inclination of the vein of Guanaxuato, is 45 or 48 degrees to the south-west. We have already stated, that it has been wrought for a length of more than 12,000 metres ; and yet the enormous mass of silver which it has supplied for the last hundred years, sufficient of itself to produce a
* Between the ravins of Sechd and Acabuca, the banks of the caliche have the same direction and the same inclination as the strata of clay slate. + 492 feet. Trans.
Or N 52o. W.
change in the price of commodities in Europe, has been extracted from that part of the vein alone contained between the pits of Esperanza and Santa Anita, an extent of less than 2600 metres.* In this part we find the mines of Valenciana, Tepeyac, Cata, San Lorenzo, Animas, Mellado, Fraustros, Rayas, and Santa Anita, which at different periods have been very highly celebrated.
The veta madre of Guanaxuato, bears a good deal of resemblance to the celebrated Spital vein of Schemnitz, in Hungary. The European miners, who have had occasion to examine both these depositories of minerals, have been in doubt whether to consider them as true veins, or as metalliferous beds (erz-lager). If we examine the veta madre of Guanaxuato only, in the mines of Valenciana or Rayas, where the roof and the wall are of clay slate, we might be tempted to acquiesce in the latter opinion ; for far from cutting or crossing the strata of the rock (quergestein), the veta has exactly the same direction and the same inclination as its strata ; but can a metalliferous bed which has been formed at the same period as the whole mass of the mountain in which it is to be found, pass from a superior to an inferior rock, from porphyry to clay slate? If the veta madre were really a bed,
* 8529 feet. Trans.
we should not find angular fragments of its roof contained in its mass, as we generally observe on points where the roof is slate charged with carbon, 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 successively 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 southeast 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, from the shafts of Guanaxuato, to the Cerro de Buena Vista and the Cañada de la Virgen, it traverses clay slate (thon schiefer). Its magnitude varies like that of all the veins of Europe.
* 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 argillaceous, or micaceous slate.
When not ramified it is generally from 12 to 15 metres * in breadth; sometimes it is even contracted t to the width of half a metre ; 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 rami. fication, and of the breadth of 7 metres S, from the surface of the ground to the depth of 170 metres. Il 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. 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 drag one another, as at Valenciana near the pit of San Antonio, at a depth of 300 metres **, the vèin contains immense riches on an extent (puissance) of more than 25 metres. tt In the pertinencia de Santa Leocadia, four branches are observable.
* From 38 to 48 feet. Trans.
† At the place of assemblage of the pit of Santo Christa de Burgos, in the Mine of Valenciana.
19 inches. Trans. § 22 feet. Trans. Il 557 feet. Trans.
164 and 196 feet. Trans. ** 984 feet. Trans. # 81 feet. Trans.