DALY. — THE NATURE OF VOLCANIC ACTION. 57 



the injected boily is tightly roofed, the gases continue to rise until the 

 growing gas-tension at the upper levels stops dill'usion. 



Genetic Classification of Volcanic Gases. 



It will conduce to clearness if a brief statement is here made as to 

 the absolute necessity of distinguishing the different classes of volatile 

 materials which are associated with igneous activity. These fluids are 

 either magmatic ox phreat'tcA^ Phreatic fluids are of atmospheric or 

 oceanic origin, and include indoi^e waters, and also those which Lane 

 has called connate (contemporaneous) waters, because trapped in sedi- 

 ments at the time of their deposition. As indicated by Suess, explo- 

 sions due to the heating of phreatic fluids by intrusive magma have 

 occurred without the ejection of true lava, either fluent or pyroclastic. 



Magmatic fluids are those actually dissolved in magma or emanating 

 therefrom. Those of primary origin and reaching the earth's surface 

 for the first time are of the juvenile class. The magmatic fluids of 

 secondary origin, that is, those absorbed from country-rock formations, 

 have been called resurgent^'' Resurgent fluids may enter the magma 

 either as constituents of assimilated country-rock or by independent 

 solution. 



Although only magmatic fluids are important in the present connec- 

 tion, it is useful to review, in tabular form, the whole group of gases 

 and vapors which are engaged in volcanic and subvolcanic activities. 



^Juvenile ■< 

 Magmatic fluids 



(volcanic ; in- 

 ternal). 



^Resurgent 



^Emanations directly from abyssal injec- 

 tion. 

 Emanations from primary solid abyssal 



country -rock. 

 fVadose and connate fluids absorbed in 



the syntectic process. 

 Vadose fluids absorbed independently of 

 ^ rock assimilation. 

 Phreatic fluids (subvolcanic ; ( Vadose. 

 external) ( Connate. 



The resurgent fluids may possibly do something toward keeping a 

 ventoi)en, but their volatilization means the partial lowering of tempera- 

 ture in the magma, so that their abundance in a conduit inii)Iie.s 



" Cf. E. Suoss, Da.s Antlitz rlcr Erdo, Bd. .3, 2to Il-ilfto, Wicn and U'ipzig, 

 V.\m, p. f)."!. 



" R. A. Daly, Amer. Journ. Science, 26, 48 (1908). 



