Upon the basis of geologic occurrence and properties, natural gases are classified as (1) those of igneous or inorganic origin and (2) those of sedimentary or organic origin. This paper treats of the variation in chemical constituents of natural gases of igneous origin with their mode of geologic occurrence. The gases occur with mineral waters from volcanic rocks, granite, and sedimentary formations, and having direct or indirect relations to the igneous activity. Hydrogen, hydrogen sulphide, carbon dioxide, and nitrogen contents of volcanic gases have close relations to the stage of volcanic activity. Oxygen and carbon monoxide are generally negligible. Helium content has no direct relation to the igneous activity, but it may have some relation to the conditions of sedimentation of the gas-bearing formations. No definite relation between helium content and radioactivity of gases is found.