GEOLOGY OF MINING DISTRICTS. 135 pyrites, destitute of garnets, and diallage metalloid (schillerspath) mixed with talc and steatite; hornblende-slate; syenite, or a granular mixture of much darkish-green hornblende, yellowish quartz, and a little lamellar and white feldspar. This syenite splits into very thin layers; the quartz and feldspar are so irregularly spread, that they sometimes form small veins in a paste of hornblende. The syenite is the largest of these eight interposed beds, of which the direction and inclination are exactly parallel to that of the whole rock; it is more than 180 feet thick; and as I saw in the deepest working of the mine (planes of San Bernardo), at 172 fathoms below the bed of syenite, carburetted clayslate occurring again, identical with that in which new shafts are beginning to be sunk, no doubt can remain that hornblende-slate alternating twice with serpentine, and serpentine alternating probably with syenite, form beds subordinate to the great mass of clay-slate of Guanaxuato. The connexion which we have just remarked between hornblenderocks and serpentine, is found in other parts of the globe, in formations of euphotide of different ages; for instance, at Heidelberg near Zell in Franconia; at Keilwig in the northern extremity of Norway; at Portsoy in Scotland; and at the island of Cuba, between Regla and Guanavacoa*. * The syenite here spoken of will best be understood in Cornwall, as a species of elvan; which term, however, more properly belongs to what is now called porphyry.—J. T. I saw no remains of organized bodies, nor beds of porphyry, grauwacke, nor lydian stone, in the transition clay-slate of Guanaxuato, which is the rock richest in silver that has hitherto been found; but this clay-slate is covered in conformable position in some places by transition porphyries very regularly stratified (los Alamos de la Sierra); in some by greenstone and syenites alternating thousands of times together (between Esperanca and Comangillas); and in others, either by a calcareous conglomerate or by transition limestone of a blueishgrey colour, mixed with clay, and fine-grained (ravine of Acabuca), or by red sandstone (Marfil). These relations of the clay-slate of Guanaxuato, with the rocks which it supports, and some of which (the syenites) appear first as subordinate beds, suffice to place it among the transition formations; above all, they justify this result in the opinion of those geognosts who are acquainted with the observations which have been recently published on the intermediary formations of Europe. With respect to lydian stone, there can be no doubt that it is contained on some points not yet explored in the clay-slate of Guanaxuato; for I found the former substance frequently imbedded in large fragments in the ancient conglomerate (red sandstone) which covers the clay-slate between Valenciana, Marfil, and Cuevas. Thirty miles to the south of Cuevas, between Queretaro and la Cuesta de la Noria, in the middle of a Mexican table-land, a transition clay GEOLOGY OF MINING DISTRICTS. 137 slate appears beneath the porphyry, darkish-gray, and passing both to siliceous slate (schistoïd jasper kiesel-schiefer) and lydian stone. Many fragments of this latter substance are found near the Noria scattered in the fields. The rocks with argentiferous veins of Zacatecas, and a small part of the veins of Catorce, according to the report of two wellinformed mineralogists, MM. Sonneschmidt and Valencia, also traverse transition clay-slate, which contains true beds of lydian stone, and which appears to rest on syenites. This super-position would prove, according to what has been observed of the great shaft of Valenciana, that the clayslates of Mexico constitute (as at Caucasus and in the Cotentin) the same formation with the transition syenites and euphotides, and that perhaps they alternate with the latter rocks. It is in porphyries and porphyritic greenstones, that at the north of the equator, in Mexico and in Hungary, the immensely rich gold and silver mines have been discovered; for although the metalliferous rock of Schemnitz (saxum metalliferum of Born) may perhaps be posterior to the transition limestones, containing some indistinct organic remains, this position, in the opinion of a celebrated geognost, M. Beudant, is too uncertain to separate formations so closely allied as those of New Spain and Hungary. The syenites with zircons, the transition granites and porphyries of Norway, which MM. de Buch and Hausmann have made us acquainted T with, are not only posterior, (Stromsoë, Krogskoven) to grauwacke and a clay-slate that alternates with orthoceratite-limestone, but these rocks also cover (Skeen) immediately a quartzite (quartzfels) that represents grauwacke, and reposes on a black limestone destitute of alternating beds of clay-slate. The property which certain porphyries and porphyritic syenites possess of being eminently metalliferous, ought not, I think, to oppose the union of the rocks of Mexico, Hungary, Saxony and Norway. The ores of gold and silver do not form contemporary beds, but are veins of extraordinary size. Some transition porphyries, several of which we should be tempted to place among trachytes because they contain true beds of phonolite with glassy feldspar, participate in these mineral riches, which, among the rocks posterior to primitive formations, were long thought to be found exclusively in carburetted and micaceous clay-slate, grauwacke, and transition limestone. There exist in the same regions groups of porphyries and syenites, very analogous, in their mineralogical composition and their position, to the rocks containing the rich mines of Schemnitz or New Spain, and which, nevertheless, are found entirely destitute of metals. This is the case with almost all the transition porphyries of South America. The great workings of Peru, those of Hualgayoc or Chota, and Llauricocha or Pasco, are not in porphyry, but mountain limestone. The famous Cerro del Potosi, in the republic of Buenos-Ayres, GEOLOGY OF MINING DISTRICTS. 139 is composed of clay-slate (transition?) covered by porphyries that contain disseminated garnets. If the great argentiferous and auriferous deposits which have formed for ages the wealth of Hungary and Transylvania, are found solely amidst syenites and porphyritic greenstones, we must not thence conclude that the case is the same in New Spain. The Mexican porphyries no doubt offer insulated examples of immense riches. At Pachuca, the pit of del Encino alone furnished annually, during a long time, more than 18,500lbs. troy of silver; in 1726 and 1727, the two workings of La Biscaina and Xacal gave together 333,000lbs. troy; that is, almost twice as much as all Europe and Asiatic Russia produced in the same interval. These same porphyries* of Real del Monte, which are connected by their upper beds with porphyritic trachytes and pearl-stone, with obsidian of Cerro de las Navajas, furnished, by the working of the mine of La Biscaina, to the Count of Regla, from 1762 to 1781, more than 3,384,000. These riches, however, are still inferior to those which are drawn in the same country from transition formations which are not porphyritic. The veta negra of Sombrerete, which traverses a compact limestone containing nodules of lydian stone, has furnished the example of the greatest abundance of silver which has been observed * Porphyry is what is called Elvan in Cornwall, where it is usually found to run in channels or courses in the killas or clay-state; in Mexico it forms larger masses.-J. T. |