In their description of the post-mine leaching of galena and marmatite at Broken Hill, Messrs. Garretty and Blanchard1 find themselves unable to account satisfactorily for the fact that the more usual order of solubility of these two minerals is here reversed, and that the galena is leached more rapidly than the marmatite. An explanation of this somewhat unusual phenomena is perhaps provided by the recent work of Garrels.2 Garrels showed that the presence of Cl' ions in Pb-Zn solutions, in concentrations in excess of 01 N, rendered the zinc less soluble than the lead. It seems possible that Cl' ions may be present in the underground waters of the North Broken Hill Mine in sufficient concentration to cause such reversal in the relative solubility of the lead and zinc sulphides.Thus the analysis of "water believed to be free from contamination by mine drainage" given by Messrs. Garretty and Blanchard (p. 158 of their paper) is as follows:Total solids in solution 878 grains per gallon...164 grams of Cl per gallon is equivalent to 2 3394 grams of Cl' per litre, which is equivalent to 0066 N.This concentration is approaching a value sufficient to cause a reversal of the relative solubility of zinc and lead salts; and it seems probable that there will be parts of the ore-body where locally the Cl' concentration will greatly exceed this figure. At such points the lead salts would be more soluble than the zinc salts, so that the galena would be leached more rapidly than the marmatite. With the dilution of such Cl'-rich waters with water containing less Cl', the Cl' concentration would fall below 01 N, and the lead would be reprecipitated, while the zinc would stay in solution, as, for example, in the water analysis quoted. The other point on which the authors speculate is the reason why the Northern Ore-body in the mine has undergone greater leaching than the...