From Introduction: "The presence in the Cripple Creek district, Colo., of irrespirable gas or gases, generally known to consist mainly of nitrogen and carbon dioxide, has been recognized by members of the mining industry in that region almost since underground mining was begun there; the occurrence origin, and possible control of these gases have been discussed in several articles cited at various places in this report. Similar gases have also been studied in other metal-mining districts, as later discussed."