Mineralogy of South America. 133 



As a rule I have found by observation that the gold is 

 purer in proportion as it is further from its source, provided 

 always that, in making this comparison, equality of size of the 

 particles is also attended to; for since the larger particles can- 

 not be so much acted upon, we naturally find them in general less 

 pure than the smaller scales associated with them in the same 

 locality. 



Tin. — The occurrence of this metal in its metallic condition 

 in nature is doubtful; and although it has been reported as 

 occurring in several parts of the world, yet the evidence is on 

 the whole not altogether satisfactory or conclusive, and I pro- 

 pose giving here an account of my examination of the cases in 

 which native tin has been reported in Bolivia. 



In the year 1859, Mr. Falkenheimer of La Paz showed me 

 two rather large pieces of metallic tin, enclosing (as if from 

 having been poured out over or amongst it) rocky matter ; these 

 had been found at Oruro in Bolivia, and there was no reason to 

 doubt the accuracy of the particulars of the occurrence. The 

 tin was found nearly pure on examination, and externally covered 

 with a coating of oxide. I discovered, however, that the sands 

 of the river on the banks of which it was found contained abund- 

 ance of Cassiterite, and preferred explaining its occurrence as 

 probably due to some of this Cassiterite having been reduced on 

 occasions of forests being on fire ; but as such fires not unfre- 

 quently have been known to arise from a stroke of lightning, and 

 so were not the effect of human agency, it might be a point for dis- 

 cussion whether tin so produced could be entitled to bear the 

 name of native tin, and rank as a mineral*. 



Subsequently, however, Mr. Philip Kroeber sent me, from the 

 gold-washings of the Playa Gritada, specimens of metallic tin, 

 which he informed me were invariably found associated with 

 the gold left in the washing-apparatus (lavadero) ; and I re- 

 solved on my visit to that locality in 1862 to examine carefully 

 into the matter. No tin ores were known to occur there when 

 I arrived f, but upon inquiry I found that since the year 1846 

 rosary pumps had been introduced to keep the water out of the 

 excavations, and that the beads of these pumps were in later 

 years formed of spheres of cast tin brought some three or four 

 hundred miles from Oruro. I felt at first quite disposed to 

 attribute the tin to some accident connected with this source. 



I must admit that a closer examination of the facts of the 



* The argument for considering it a mineral would apparently be quite 

 as strong as in the case of Struvite, which was first found in the founda- 

 tions of the church of St. Nicolas at Hamburg. 



t I found them (Cassiterites) in abundance, however ; but, from igno- 

 rance, they had invariably been thrown away, not having even been known 

 to contain tin. 



