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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov. 21, 



The rocks from this mine to Pongo, and from that place down to 

 Unduavi, consist entirely of more or less arenaceous shales, thin- 

 bedded and containing frequently indistinct traces of fossil forms. 

 As seen in the section, these slates are dislocated by three faults, which 

 faults are filled with metallic matter, and form the veins on which are 

 worked the mines " Delphina," " Mercedes" (60° N.E. strike, 85° E. 

 dip), and " Pilar," producing argentiferous galena more or less an- 

 timonial, for the supply of the furnaces at San Felipe. 



In M. D'Orbigny's map, the greater part of these slates are 

 coloured as granite ; and, according to him, Pongo is situated in the 

 midst of the granite, which in his map here forms a broad band, 

 constituting the centre of this range of the Andes. As seen in the 

 section, no trace of granite visible to me occurs here ; and, in fact, no 

 granite whatever is crossed in the direct line of section from La Paz 

 to as far east of Unduavi as I examined. The small outburst of 

 granite seen at Silla Tuneari does not occur in this line ; and I have 

 purposely deviated the section from the direct line in order to show 

 it. Granite is also met with in the Nevado of Chucura, at Yoja, and 

 at Takesi (to the south of this section) ; but it does not form the 

 continuous band seen on M. D'Orbigny's map, which appears in 

 this, as in many other instances, to have been coloured from 

 imagination*. 



The granite of this chain of the Andes shows itself at various 

 localities, apparently isolated one from anotherf, and appears to be 

 of the same age as the auriferous granites of the rest of the world, 

 with which it is identical in mineralogical and chemical composition, 

 being composed of white orthoclase, colourless quartz, and black or 

 white mica, and containing frequently spots of iron-pyrites, which 

 sometimes, as at Silla Tuneari, stains the granite of a brown colour 

 from oxidation. The gold in this is found in the quartz, or along 

 with the pyrites ; and it would occasionally, at least, appear that 

 this rock is more auriferous in proportion as it contains more pyrites. 

 As the whole of this Silurian formation is eminently auriferous, and 

 contains everywhere frequent veins of auriferous quartz, usually 

 associated also with iron-pyrites, a study of the occurrence of these 

 gold-veins leads me to attribute all such veins to the proximity of 

 granite, and to regard the veins of quartz, iron-pyrites, &c. as having 

 been directly injected from the mass of granite itself. We know 

 that, during the cooling and solidification of granite, the quartz 



* In the map which accompanies this memoir (PI. I.) the granite is not 

 separately coloured, on account of its forming here a subordinate part of the 

 Silurian district. It being difficult to get details into so small a scale, the Devo- 

 nian and Silurian, along with the granite rocks which disturb tbem, are coloured 

 with one tint. I must also confess that, without more data, it would be im- 

 possible to do otherwise without falling into errors similar to those of M. 

 D'Orbigny. 



t I must observe that I am not alluding to the great granitic range which 

 occurs, according to M. Pissis, still further to the east, near Coroico. I, unfor- 

 tunately, was unable to pursue my section further, from the rainy season having 

 rendered the rivers impassable: an attempt to ford the river below Unduavi 

 having residted in the loss of two animals with all the baggage, made it more 

 prudent to retrace my steps, however unwillingly, to La Paz. 



