Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/356

 ditions that should lead to such a result by after-reduction of the silver salt.

A specimen from the "Last Chance" reef in the New-Zealand Gully, near Rockhampton, shows specks of metallic gold disseminated through a mass of chloride of silver. It has not yet been examined to see if any portion of gold may be in a form soluble in hyposulphite of soda.

From 7 tons of the vein- stuff of the " Last Chance" 240 ounces of gold were washed. The harder portion, which would not " puddle down " when crushed and amalgamated, yielded a bar of metal very pale in colour, and with a much higher percentage of silver than the washed-out gold from the same reef.

This was due to the abundance of " poison-stone," as the owners called the horn silver, which remained among the harder portion of the reef sent to the stampers.

Though this and other cases of a similar bearing would suggest that both gold and silver were originally in solution as chlorides (the latter by the solvent power of an alkaline chloride), yet the difficulty of precipitating the two metals together from such a solution has yet to be overcome. The more constant association of native gold with pyrites would also draw our attention in another direction.

A series of experiments now in hand may throw some light on this complicated question ; and when completed they will form the material for a further communication to this Society.

Whatever may have been the solvent and precipitant of the nobler metals in the auriferous vein-stones associated with trap-intrusions, all other but hydrothermal action may safely be eliminated, the very nature of the reefs, composed as they are of alternating layers or of a promiscuous mixture of quartz, calc-spar, pyrites, &c., affording unmistakable evidence on this point. The gold also contained in the trap-dykes themselves is always accompanied by pyrites — both, in my opinion, hydrothermal products separating out during the cooling down of the trap-intrusions.

Auriferous lodes, occurring in areas where hydrothermal action has attended trap-disturbances of a special character in Queensland, are generally thin — to be estimated by inches rather than by feet ; but, taken as a whole, they are far richer in gold than those enclosed by sedimentary rocks.

The yield of the principal Gympie reefs for the year 1869 was 11,996 tons of quartz for 76,870 ounces of gold, being an average of 6 ounces, 8 dwts., 4 grains per ton, the highest return of perhaps any gold-field in the world. The cost of mining such reefs, however, is much greater than where the vein-stones are better defined and of a good average thickness throughout.

The following Table, showing the thickness and yield of most of the reefs in the Rockhampton district, affords a fair representation of the character, thickness, and general conditions of the auriferous lodes found in the other mining- districts included within Palaeozoic