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XXII, Researches on the Mineralogy of South America. 

 By David Forbes, F.R.S., §-c* 



II. 



-KTATIVE Gold.— The valley of the Rio de Tipuani, to the 

 -^ eastward of Sorata, in the department of La Paz, Bolivia, 

 is in all parts extremely auriferous, and contains some of the 

 most productive gold-washings in South America, if not in the 

 whole world. 



Since the independence of the Bolivian Republic and the 

 abolition both of negro slavery and the forced labour of the 

 Indian tribes, the greatest difficulty has been in procuring 

 labourers ; and in consequence of this scarcity, in combination 

 with the unhealthiness of the climate, the gold- washings f in 

 many parts of the valley, previously worked, have been aban- 

 doned, and those still remaining in operation are worked upon 

 a scale far from commensurate with their magnitude and rich- 

 ness. 



In 1862, when studying the geology of this part of Bolivia, I 

 examined the valley of the Tipuani River, from its source in the 

 great mountain Illampu (the highest of the Andes of South 

 America) down to the river Mapiri, into which the Tipuani dis- 

 charges itself, and I now present the results of a chemical exa- 

 mination of the native gold found in the sands of the river at 

 various parts of its course. 



Gold from Ancota. — This gold- washing establishment, pertain- 

 ing to Don Ildefonso Willemil, is situated on the other side of 

 the river, and a little above the village of Tipuani J ; and at present 

 its operations are confined to washing the banks which confine 

 the course of the river, without working the actual bed of the 

 river itself. 



These banks, which in ancient times had formed the bed of 

 the river, are composed of rock-debris, coloured deep red from 

 the large amount of sesquioxide of iron which they contain. 



The fragments of rocks in them are principally ferruginous 

 clay-slates and greywacke of the Silurian age, with some meta- 

 morphic schists and a white granite §, which latter rock appears to 



* Communicated by the Author. 



f These washings were first known to the Spaniards in the year 1581, 

 but used by the Indians long before. 



J The name Tipuani comes from " Tipa," the name of the Dragon's- 

 blood tree in the Aymara language, as numbers of these beautiful trees are 

 seen growing on its banks. 



§ This granite, composed of white orthoclase felspar, colourless quartz, 

 and black or colourless mica, is the same as mentioned in the former part 

 of this paper as occurring at Illampu; it is there supposed to be of Middle 



Phil Mag. S. 4. Vol. 29. No. 194. Feb. 1865. K 



