housands of years after the deposition of the sulphide ores had ceased and the oxidation had begun.

If the leaching of the Leadville porphyry has not resulted in the formation of alkaline sulphide solutions, and the ore has come from the porphyry in the condition of carbonate of lead, chloride of silver, etc., then the nature of the deposition was quite different from that of the similar ones of Tybo, Eureka, Bingham, etc., which are plainly gossans, and indeed is without precedent.