The work of previous investigators has shown that certain relatively stagnant lake and canal waters and some shallow ground waters are highly charged with H2S as a result of the reduction of soluble sulphates by certain anaerobic bacteria. Recent work by the present writer and his collaborators has shown that waters rich in H2S and carrying sulphate-reducing bacteria are associated with petroleum in several of the largest producing oil fields of Illinois and California. If solutions carrying metals in balance with the sulphate, chloride, or bicarbonate radicals come in contact with such relatively stagnant H2S-bearing waters, precipitation of the metals as sulphides must result. Such a mechanism of precipitation is suggested as a hypothesis deserving of critical consideration in future studies of certain types of sulphide ores such as the "Kupferschiefer," the copper deposits in red beds and other sandstones, and lead and zinc deposits of the Mississippi Valley type.