Mineralogy of South America, 131 



The minerals associated with the native gold are oxide of tin 

 in considerable quantity, black tourmaline, garnet, rounded and 

 highly polished nodules of hsematite, pseudomorphous crystals 

 of oxide of iron after iron pyrites, a rose-coloured mineral, appa- 

 rently one of the topaz-group, and some small red fragments, 

 apparently of ruby ; metallic tin also occurs, as will be noticed 

 further on. The rock of the bed is generally, if not always, clay- 

 slate, and the loose soil above it is composed of boulders of the 

 granite before described, often of immense size, along with Silu- 

 rian slates, greywacke, and metamorphic schists of the immediate 

 neighbourhood. 



The gold is generally found perfectly free from any rock 

 matrix whatever ; and I only succeeded in finding a single and 

 minute specimen which contained any gangue, and in that case 

 it was colourless quartz. 



The gold is very regular in size, being best compared in size 

 and shape to melon-seed, and of a very fine colour; occasionally 

 larger nuggets have been found, but more as an exception, the 

 size of a melon-seed being the rule • but comparatively little 

 gold dust is associated with it, and of this most is carried off by 

 the water in the imperfect arrangements for washing which are 

 at present employed. 



The specific gravity, taken on 428*49 grs. of the rather 

 larger pieces, was found to be 17*906 at 60° F., whilst some of 

 the smallest pieces, determined separately, gave 17*84 at 60° F. 

 The analysis was conducted as in the last case, and afforded 



Gold 93*51 



Silver 6*49 



100-00 



Gold from Romanplaya. — Still higher up the river, probably 

 some ten miles above the last-mentioned workings at the Playa 

 Gritada, are the gold-washings of Romanplaya, in which the 

 covering of diluvial gravel &c. was found to be fully 60 feet in 

 depth above the solid rock on which the gold lies, and appears 

 to be composed of the same rocks as at the Playa Gritada. 



The gold in appearance much resembled the last, and its spe- 

 cific gravity, taken at 60° F., using an amount of 150*69 grs. in 

 particles of about the size of a small melon- seed, was found to 

 be 18*672. 



On analysis, its percentage composition was found to be : — 



Gold . . . 94*189 

 Silver . . . 5*811 



100*000 

 K2 



