232 Principal Silver Mines in Mexico and South America. 



minerals, in the veins near the surface, are in a state of decomposi- 

 tion, and mingled with much oxide of iron, like the pacos of Peru. 



IV. Pasco mines. 



These mines are situated south west of the city, of Mexico. The 

 most ancient rock is argillaceous slate, and this is covered by a for- 

 mation of porphyry, which contains common and vitreous feldspar, 

 and beds of blackish brown pitchstone ; and again, the porphyry is 

 covered by a bluish gray compact alpine limestone, which is often 

 porous, and contains subordinate beds of gypsum and argillaceous 

 slate, and also univalve shells. The limestone is overlaid by a sand- 

 Stone, with a calcareous cement. 



The veins traverse both the limestone and argillaceous slate, but 

 they are richest in the limestone. Some of the veins have a breadth 

 of thirteen feet, but the ores are not uniformly disseminated in their 

 gangue, and in general their produce is very variable. 



These mines must not be confounded with the Pasco mines of 

 Peru, which are situated about thirty or forty leagues north of Lima. 

 For an account of these, the reader is referred to the American 

 Journal of Science, Vol. XVII, p. 43, and Mier's Travels in Chili, 

 II, p. 432, et seq. 



The principal mining districts of Peru, are those of Pasco, Cho- 

 ta, and Huantajaya. The veins of silver ore are very numerous, 

 but nearly all the silver produced is from a few mines. The PascQ 

 or Yauricocha mines have been already referred to, in the American 

 Journal of Science, Vol. XVII, p. 43. 



The mines of Chota are situated on the Andes, at about 7° S. lati- 

 tude. The principal ones are those of Gualgayoc and Micuipampa, 

 and their discovery dates from 1771, but the ancient Peruvians had 

 explored silver veins in the vicinity. The veins, composed of sul- 

 phuret of silver, red silver, and native silver, traverse sometimes al- 

 pine limestone, and sometimes hornstone, which forms subordinate 

 beds. The upper part of the veins is a red earthy ferruginous mass, 

 containing silver, and called pacos. In some places, large quantities 

 of ore are found on the surface of the earth. In a small plain, call- 

 ed la Pampa de JVavar, wherever they have removed the turf, they 

 have found sulphuret of silver and native silver, adhering to the roots 

 of the grass, and often the silver is in irregular masses, as if the melt- 

 ed metal had been poured upon soft clay. 



