latter bear the greatest analogy to the porphyry slate (porphyrschiefer) of the mittelgebirge of Bohemia. One would be tempted to reckon them among the rocks of trappformation, 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 amphibole is almost as rare in them as quartz and mica. The direction and inclination of these beds, are the same as those of the clay

slate.

On the southern slope of the sierra, and generally at smaller elevations than that at which porphyry is found, in the plains of Barras, and Cuevas, especially between Marfil, Guanaxuato, and Valenciana, the clay-slate is covered with free-stone of very old formation. This free-stone (urfelsconglomerat) is a brescia with clayey cement, mixed with oxide of iron, in which are imbedded angulous 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 agglutinated by an ochreous 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 brescia, having large flints. This formation of old free-stone is the same with that which appears at the surface in the plains of the river Amazon, in South America, and which, in Switzerland, rises to more than a thousand metres of absolute height, in the Oltenhorn and the Diablerets, has no regularity in the direction of its beds. Their inclination is generally opposite to that of the strata of clay slate. Near Guanaxuato, the formation of freestone is at the back of the porphyry of the buffa; but near Villalpando, the porphyry itself serves for base to the antient brescia, which appears at the surface at an absolute height of 2600 metrest.

We must not confound the brescia which contains imbedded fragments of primitive and transition rock, with another freestone, which may be designated by the name of felspar agglomeration, and which, at the mountain of la Cruz de Serena, is superimposed to the

*From 26 to 29 feet. Trans.

† 9842 feet. Trans. 3842 (see emrata)

8529 feet. Trans.

antient brescia (urfels conglomerat) that is consequently of a more recent formation. This agglomeration (lozero) from which the finest hewn stone is manufactured, is composed of grains of quartz, small fragments of slate, and felspar chrystals, partly broken, and partly remaining untouched. These substances are connected together by an argilo-ferruginous cement. Probably the destruction of porphyries has had the greatest influence on the formation of this felspar freestone. It contrasts with

the freestone of the Old Continent, in which some chrystals of grenats and amphibole have been found, but never as far as I know, felspar 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 brescia (trümmer-porphyr). Near Villalpando, about thirty very thin banks of slate clay (schiefer thon) of a blackish brown colour, alternates with the felspar agglomeration.

These formations of old freestone of Guanaxuato, serve as bases 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 mineral of manganese, either earthy or radiated. This calcareous stone which from its even and almost conchoidal fracture, resembles the formation of jura, is covered in some points with banks of fibrous gypsum mixed with hardened clay.

We have thus enumerated the various rocks which repose on the clay slate of Guanaxuato, and which are on the one hand secondary formations of freestone, limestone, and gypsum, and on the other formations of porphyry, syenite, serpentine and amphibolic 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 porphyry mountains exhibit the most whimsical forms from the manner in which they are torn asunder; and to the westward we discern 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 during an extent of more than twenty square leagues serves for a base to beds of syenite which alternate with transition

greenstone. These beds are in general from four to five decimetres 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 amphibole. 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 theory of the origin of veins, laid down by Mr. Wernert. Near Chichimequillo, a columnar porphyry seems to repose on syenite. It is covered with basalt and basaltic brescia, from which the springs of which the temperature is 96° 3 of the centrigrade thermometer, have their source.

It remains for me to give an account of two partial formations which occupy only a very small extent: a compact limestone (el caliche) of a blackish grey, belonging perhaps to

*From 15 to 19 inches. Trans.

+ Neue Theorie von der Entstehung der Gänge, 1791, p. 60. 205° of Fahrenheit. Trans.