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x1.) KINGDOM OF NEW SPAIN. 177 the beds with very thin folia which it contains, and which are surcharged with carbon, appear to approximate it to transition clay slate. These beds (oja de libro) are for the most part found near the surface * ; but sometimes they are visible t at considerable depths. On digging the great pit (tiro general) of Valenciana, they discovered banks of syenite of Hornblend slate (Hornblend schiefer) and true serpentine, alternating with one another, and forming subordinate beds, in the clay slate. This extraordinary phenomenon of a syenite alternating with the serpentine, is also to be seen in the island of Cuba, near the village of Regla, where the latter rock abounds in schillerspar (schillersputh.) The same clay slate of Guanaxuato which is observed at the bottom of the mine of Valenciana, re-appears at the surface, eight hundred metrest, higher up on the ridge of the Sierra de Santa Rosa, but I doubt whether it has ever been found at greater elevations. These strata are very regularly directed h. 8 to 9 of the miner's compass §; they are inclined from 45 to 50
* In the mine of Valenciana.
Or from South-East to North-West. I have been struck ever since 1791, with this great law of the parallelism of the beds, which are discovered in immense extents of country, and which may be regarded as one of the most curious phenoVOL. II,
degrees to the south west. This is the di. rection of the greatest part of the very old rocks of Mexico.
Two yery different formations repose 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 freestone 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; and 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 an 6 ; my Tableau Geologique de l'Amerique Meridie onale (Journal de Physique 1800 ;) and my Geographic 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 beds, repose 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 chrystals 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, inchased in a mass, which sometimes passes into the petrosilex jadien,' and sometimes into the pholonite or klingstein of Werner. The
* Being a scholar 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 feuerstein (pyromaque). The hornsteine of the German mineralogists are, the Quartz-agathes, grossier et wyloides of M. Haüy, the neopetres of Saussure, and the siled cornés of M. Brogniart. This note appeared to me indispensable, on account of the confused synonomy of the de. nominations protosilex, pierre de corne, and roche de corne.
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 metrest 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.
8529 feet. Trans.