228 



Principal Silver Mines of Mexico and South America 



I. Mines of Guanaxuato. 



There is one circumstance in the situation of the immense silver 

 vein, called "la veta madre," which is very remarkable, viz. that it 

 is parallel to the strata containing it.* This vein has been, recog- 

 nized over a length of fourteen thousand yards, or about nine miles, 

 and has a breadth of about forty five yards. Most of the nineteen 

 mines wrought on this vein, are very productive. 



The materials composing it, are native silver, sulphuret of silver, 

 red silver ore, native gold, sulphurets of lead, zinc, iron and copper, 

 carbonates of iron and lead, and some gray copper. The stony ma- 

 terials are quartz, calcareous spar, pearl spar, feldspar, chalcedony 

 and fluor spar. The vein of Guanaxuato passes through argillace- 

 ous slate, and also through porphyry. The argillaceous slate ap- 

 pears to be the most ancient rock of the district. It sometimes 

 passes into talcose and chloritic slates, and beyond, it is seen re- 

 posing upon the granites of Zacatecas and Penon Blanco. This 

 slate contains subordinate beds of sienite, hornblende slate, serpen- 

 tine, and greenstone, and it is observed that the sienite contains veins 

 of greenstone , and the greenstone veins of sienite. 



Upon the argillaceous slates repose two different formations, viz. 

 porphyry and " gres ancien,"f the former constituting the elevated 

 peaks, while the latter fills the ravines and low grounds. 



The porphyry presents gigantic masses, like ruins, and precipitous 

 escarpments, of from one thousand to one thousand five hundred feet 

 in height. This porphyry is, in general, of a greenish color; its 

 paste is variable, being in the oldest, compact feldspar or petro- 

 silex — in others, it approximates to jade, or to phonolite. 



The newer porphyries contain glassy feldspar, and much resem- 

 ble the porphyritic slate (porphyrschiefer) of Bohemia. Enormous 

 concentric balls of these porphyries are sometimes seen reposing 

 upon isolated rocks. The aggregate of the characters of these 

 rocks, indicates that they belong to the class of trap rocks, except 

 that rich gold mines have been found in them at Villalpando. 



# For evidences of its being a vein, and not a bed, see Humboldt's Essai sur la 

 Nouvelle Espagne, t. iii, p. 395, and Ann. des Mines, t. xxxi, p. 323. 

 t Annates des Mines, t. xxxi, p. 325. 



