Principal Silver Mines in Meooico and South America. 229 



These porphyries have the same direction and dip as the argilla- 

 ceous slate, the line of bearing being from north west to south east, 

 and the dip from 45° to 50° to the south west. 



The "gres ancien" is an aggregate of angular fragments of quartz, 

 lydian stone, sienite, hornstone and porphyry, cemented by an argillo- 

 ferruginous cement. This rock rests upon the argillaceous slate, but 

 has an opposite inclination. There is also another grauwacke, (gres,) 

 which differs much from the last. It is composed of fragments of 

 quartz and slate, and particularly of uninjured crystals of feldspar , 

 which might cause it to be mistaken for a porphyry. M. Humboldt 

 calls it "gres," or "agglomerat feldspathique." The cement is ar- 

 gillo-ferruginous, and thin layers of shale (" schieferthon") alternate 

 with the rock. It is an excellent building material, easily dressed, and 

 is called in the country "lozero." A limestone, analogous to the Jura 

 limestone, overlies all these grauwackes. There are also local, cal- 

 careous breccias, transition limestone, and various trappean rocks. 



Upon the vein of Guanaxuato, which is the only one of the whole 

 district, there are nineteen explorations. They furnish one fourth of 

 all the silver of Mexico. The richest of these mines, and in fact 

 the richest of all Mexico, is that of Valenciana. This mine was 

 opened in the sixteenth century, and afterwards abandoned. In 

 1760, it was opened again, and wrought for several years, under 

 very discouraging circumstances, by M. Obregon, since Count de 

 Valenciana. He arrived, finally, at a rich part of the vein, and soon 

 replaced all his former losses, and many fold more. During some 

 years, the net profit of the mine was $1,200,000 per annum. 

 The three old shafts. of the mine cost the Count de Valenciana 

 $1,200,000. In 1792, it was judged expedient to construct a new 

 shaft for the extraction of the ore. It was commenced, at that time, 

 with a diameter of thirty feet, with the expectation of being able to 

 reach the vein in 1815, at the depth of one thousand six hundred 

 and fifty feet. The shaft is well executed ; the stone lining is per- 

 fect of its kind, and Humboldt considers it a model worthy of ex- 

 amination. The objects of this shaft were not only, as usual, to fa- 

 cilitate the extraction of the mineral, but also to diminish the num- 

 ber of porters. 



Vol. XXIV.— No. 2. 30 



