eee emma ee AAA AS 
METALLIFEROUS DEPOSITES. 393 
felspar is of rare occurrence, but the glassy variety 
is frequently observed in them. The rich gold mine 
of Villalpando, near Guanaxuato, traverses a por- 
phyry, of which the basis is allied to clinkstone, and 
in which hornblende is extremely rare. ‘The veins 
of Zuriapan intersect porphyries, having a green- 
stone basis, and contain a great variety of interesting 
minerals, such as fibrous zeolite, stilbite, grammatite, 
pycnite, native sulphur, fluor, barytes, corky asbes- 
tus, green garnets, carbonate and chromate of lead, 
orpiment, chrysoprase, and fire-opal. 
Among the transition rocks, containing ores of 
silver, may be mentioned the limestone of the Real 
del Cardonal, Xacala, and Lomo del Toro, to the 
north of Zuriapan. In Mexico graywacke is also 
rich in metals. 
The silver-mines of-the Real de Catorce, as well 
as those of El Doctor and Xaschi, near Zuriapan, 
traverse Alpine limestone, which rests on a conglo- 
merate with siliceous cement. In that and the 
Jura limestone are contained the celebrated silver- 
mines of Tasco and Tehuilotepec, in the intendancy 
of Mexico; and in these calcareous rocks the me- 
talliferous veins display the greatest wealth. 
It thus appears that the cordilleras of Mexico 
contain veins in a great variety of rocks, and that 
the deposites which furnish almost all the silver ex- 
ported from Vera Cruz are primitive slate, gray- 
wacke, and Alpine limestone. The mines of Potosi 
in Buenos Ayres are contained in primitive clay- 
slate, and the richest of those of Peru in Alpine lime- 
stone. Our author here observes, that there is scarce- 
ly a variety of rock which has not in some country 
been found to contain metals, and that the richness 
