24 On the Geological Distribution of Gold. 



The pyrites of Marinate-, being the prevailing compound in 

 the several veins occupying this district, is also found in the 

 other two localities. In the same district, but at a distance 

 of a few miles, the veinstone at Botafuego produces gold of a 

 higher quality or gold 90,91, silver 09,09. 



The assays of the gold produced in these veins, all of 

 which were from the same locality, and from the same 

 quality of ore, show a gradual decline in the quality of gold 

 from 18 to 9 carats; this remarkable difference in alloy origi- 

 nating from the metallic compounds by which the gold is 

 associated in the pyrites, the free gold-particles containing 

 less silver, and those with the sulphuret of lead, less gold. 



From these facts it may therefore be deduced, that if formerly 

 it were supposed that gold, in its native state, maintained a 

 determined proportion with regard to its alloy of silver ; there 

 certainly remains no further doubt about these metals being 

 isomorphous. This fact has been proved many years ago, 

 with regard to the nature of gold from the Ural mountains ; 

 carefully investigated by M. Gustave Rose. 



Returning to the supposed analogy between gold from 

 the rock and that produced from fluvial deposits, it may be 

 here proper to state the appearance and alloy of the fluvial 

 gold collected in the neighbourhood and close to the por- 

 phyries of Marmato. The mountain-ridge of this locality 

 being the most elevated point of the Supia district, and 

 situated to the north of the fluvial deposits on the Supia 

 plains and to the west of the Cauca river, might be supposed 

 to have contributed to the auriferous nature of the fluvial 

 detritus, settled at a short distance and at a lower level, but 

 such is not the case. 



Superior Fluvials, of the neighbourhood of the Marmato 

 veins (Alluvial) : — 



The streamlet Obispo, passing over the porphyry, produces 

 small lamellar gold, in well-rounded particles of clean and 

 bright surface, its quality being 20. 1-12 carats. 



The River Supia, passing over the plains ; its gold being 

 of flat form, well rounded outlines, clean and bright yellow — 

 its quality, 22. 9-32 carats. 



The border of the Cauca River, at low water, to the south — 

 gold of a small flat grain, well-rounded outline, clean and 

 bright surface — quality, 19. 25-48 carats. 



The border of the Cauca River, at low water, to the north- 

 gold in small lamellar particles, well-rounded outlines, clean 

 and bright surface — quality, 20. 11-24 carats. 



