degrees to the south-west.

This is the di

rection of the greatest part of the very old rocks of Mexico.

:

Two very different formations rest on the clay slate the one of porphyry at considerable elevations to the east of the valley of Marfil, and to the north-west of Valenciana; and the other, of old sandstone in the ravins, and table lands of small elevation.

Porphyry forms gigantic stony masses, which appear at a distance, under the strangest aspect, frequently like ruins of walls and bastions. These masses are perpendicular, and from three to four hundred metres *, elevated above the

mena of geology. I have never ceased in my writings from calling the attention of travellers to an object, with regard to which it would be easy to collect in a very short time a great number of observations. See my experiments on the irritation of the muscular and nervous fibre, (in German) vol. i. p. 8; my letter to M. de Fourcroy, dated 5 Pluviose an6; my Tableau géologique de l'Amérique Méridionale (Journal de Physique 1800); and my Géographie des Plantes, p. 117. The direction of high chains of mountains appears to have the greatest influence on the direction of the beds, even at considerable distances from the central crest. This influence is manifest in the Pyrenees, Mexico, and especially in the Upper Alps. See the judicious observations which M. Ebel, a learned mineralogist has published on this subject under the title of, On the Construction of the Chain of the Alps (in German) vol. i. p. 220; vol. ii. p. 201 215. & p. 357.

From 984 to 1314 feet. Trans.

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 felspar. 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 klingstein 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 synonomy the denominations petrosilex, pierre de corne, and roche de

corne,

of

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 † 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. ‡

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

breccia (urfels conglomerat), and is consequently 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, 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 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 Guanaxuato, 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