210 7Vi^? Geoloijjf of the. Gold Flckh of Britlslt Giiidua. 



were separately burnt to ashes in a new muffle in a furnace which had 

 never heen used for gold assaying, when the bark yielded 4*78 per cent, 

 of its weight of ash, and the wood gave only •67 per cent. Several 

 trials were made, using quantities of four assay tons of the ashes for each 

 assay, and cupelling in new muffles, whilst bv blank experiments, the 

 crucibles and the materials used were proved to be free from all traces 

 of gold. The ash of the bark yielded gold at the rate of only one grain 

 per ton of ash, whilst that of the wood yielded in the various trials 

 made from seven to ten grains of gold per ton of ash. Thus the trials 

 contirmed the results obtained l)y Dr. Lungwitz a; fully as possible, and 

 were made under conditions of strict control, which were free from the 

 objections raised by Du Bois to those under which the earlier ones were 

 made at Omai. 



Proof was also obtained that the waters of the Omai Creek contained 

 gold. A sample of a rusty deposit was sent to me which had been 

 oljtained from the steel valves of one of the pumjjs used for producing 

 hydraulic power for washing the laterite at Omai. Many millions of 

 gallons of the ci^eek water had passed through the valves, which had 

 become somewhat corroded. The deposit was found to yield gold at 

 the rate of one hundred aiid fifty-six grains to the ton. This proof, 

 however, is not as free from objections as are the concordant results 

 obtained by Dr. Lungwitz and by myself, which showed that the ash 

 of the " ironwood " trees grown at Omai yielded gold in appreciable 

 amounts. It is of course possible that the gold found in the rusty 

 deposit of the pump-valves was present in the waters in suspension, 

 whilst it is not possible for gold in that state to have obtained access to 

 the interior wood of the tree trunk. 



The fact of the solubility of gf>ld ii\ soil-water having been thus 

 proved, it is easy to understand how the concentration of the metal 

 disseminated through the basic rocks takes place during their de- 

 composition. We are not in a position to point out exactly what are 

 the precipitating agents in the laterite, but there are many well-known 

 ones which may have thrown the gold out of solution. 



Where the sources of the gold were in the mineralised masses of 

 acidic rocks similar actions of solution, and re-deposition to those 

 which affected the gold of the basic rocks caused the concentration of 

 the metal in their residuary products. 



In many of the gold-bearing districts of British Guiana placers are 

 situated at considerable elevations on the hillsides. In these the 

 laterites are in, or are very near to, their position of formation, and 

 the only processes of concentration that could have acted on them are 

 the chemical ones of solution and re-deposition, and the mechanical 

 ones of the sinking of the (juartz gravel, the gold and the heaviei' 

 I'esistant minerals through the laterite when satiu'ated with water. 

 The heavier minerals consist mainly of iron-ores, and I have shown 

 that these are, in places, markedly auriferous. 



The variations in the ])roi)ortions of gold contained in the basic 

 rocks give rise to similar variations in the amounts of the metal found 

 in the latei'ites and the concretionary ironstones. Some dej)osits of 

 laterite in situ are almost barren, whilst others are rich in gold. This 



