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surrounding plains. In the country they go by the name of Buffa. Enormous balls with concentrical layers rest on insulated rocks. These porphyries give a savage character to the environs of Guanaxuato, calculated to astonish the European traveller, who imagines that nature never deposits great metallick wealth but in mountains with round tops, and in places where the surface has a gentle and uniform undulation. This porphyry of which the Sierra de Santa Rosa is chiefly composed, is generally of a greenish colour ; but it varies very much according to the nature of its base, and the crystals which it contains. The oldest beds appear to be those of which the base is hornstone (hornstein) or compact fetspar. The most recent on the other hand, contain vitreous felspar, imbedded in a mass which sometimes passes into jade, and sometimes into the phonolite or kling stein of Werner. The latter bear the
* Being a pupil of Werner, and of the school of Freiberg, I every where name in my works Hornstein a mineral which forms transitions into quartz, calcedony, and flint (feuerstein). The hornsteine of the German mineralogists are, the Quartzagathes, grossier et xyloïdes of M. Haüy, the néopètres of Saussure, and the silex cornés of M. Brogniart. This note appeared to me indispensable, on account of the confused
of the denominations petrosiler, pierre de come, and roche de
greatest analogy to the porphyry slate (porphyrschiefer) of the mittelgebirge of Bohemia. One would be tempted to reckon them among the rocks of trapp-formation, if these same beds did not contain at Villalpando the richest mines of gold. All the porphyries of the district of Guanaxuato possess this in common, that hornblende is almost as rare in them as quartz and mica. The direction and inclination of their strata are the same as those of the clay slate.
On the southern slope of la sierra, and generally at smaller elevations than that at which porphyry appears in the plains of Barras and Cuevas, especially between Marfil, Guanaxuato, and Valenciana, the clay-slate is covered with sand-stone of an old formation. This sand-stone (urfels-conglomerat) is a breccia with argillaceous cement, mixed with oxide of iron, in which are imbedded angular fragments of quartz, Lydian stone, syenite, porphyry, and splintery hornstone. Beds containing from six to eight centimetres * in thickness alternate sometimes (near Cuevas) with other beds, in which grains of quartz are conglutinated by an ochry cement. At other times in the ravin of Marfil and
* From 2 to 3 inches. Trans.
in the road of Salgado) the cement becomes so abundant that the imbedded fragments entirely disappear, and banks of slate-clay of a yellowish brown, from eight to nine metres in thickness * alternate with breccia, composed of large flints. This old sand-stone formation is the same with that which appears at the surface in the plains of the Amazon river, in South America, and which, in Switzerland, rises to more than a thousand metres t of absolute height, in the Oltenhorn and the Diablerets, has no regularity in the direction of its beds. Their inclination is generally the reverse of that of the strata of clay slate. Near Guanaxuato, the sand-stone formation overlays the porphyry of the Buffa; but near Villalpando the porphyry itself serves for base to the old breccia, which appears at the surface at an absolute height of 2600 metres. I
We must not confound the breccia which contains imbedded fragments of primitive and transition rock, with another mass which may be designated by the name of felspar conglomerate, and which, at the mountain of la Cruz de Serena, overlays the old
* From 26 to 29 feet. Trans.
breccia (urfels conglomerat), and is sequently of a more recent formation. This conglomerate (lozero) which yields the finest building-stone, is composed of grains of quartz, small fragments of slate, and felspar crystals, partly broken, and partly in a perfect state. These substances are connected together by an argillo-ferruginous cement. Probably the destruction of porphyries has had the greatest influence on the formation of this feldspatic sand-stone. It contrasts with the free-stone of the Old Continent, in which some crystals of garnets and amphibole have been found, but never, as far as I know, fel. spar in any abundance. The most experienced mineralogist, after examining the position of the lozero of Guanaxuato, would be tempted to take it at first view, for a porphyry with clayey base, or for a porphyritic breccia (trümmer-porphyr). Near Villalpando, about thirty very thin banks of slate clay (schieferthon) of a blackish brown colour, alternate with the feldspar conglomerate.
This old sand-stone formation of Guanax. uato, serves as a basis to other secondary beds, which in their position, that is to say in the order of their superposition, exhibit the greatest analogy with the secondary rocks of central Europe. In the plains of Temascatio (at lo de Sierra) there is a compact lime
stone (dichter kalkstein) frequently full of vesicular cavities, which are coated with calcareous spar, and oxide of manganese, both earthy and radiated. This 'rock which, in its even and almost conchoidal fracture, resembles the Jura lime-stone, is covered in some points with banks of fibrous gypsum mixed with hardened clay.
We have thus enumerated the various rocks which rest on the clay slate of Guanaxuato, and which are on the one hand secondary formations of sandstone, limestone, and gypsum ; and on the other formations of porphyry, syenite, serpentine and hornblende slate. The ravin of Marfil, which leads from the plains of Burras to the town of Guanaxuato, separates as it were the porphyritic region from that in which syenite and greenstone predominate. To the east of the ravin, very steep prophyry mountains exhibit the most whimsical forms from the manner in which they are torn asunder and to the westward we see a district of which the gently undulated surface is covered with basaltic cones.
From the mine of Esperanza, situated to the north-west of Guanaxuato, to the village of Comangillas, celebrated for its hot springs, the clay slate, to the extent of more than twenty square leagues, serves for a base to beds of syenite which alternate with transition