DALY. — THE NATURE OF VOLCANIC ACTION, 101 



the basic mass or masses can only be interpreted as due to injection. 

 Branco dates the intrusion of the Ries " laccoHth " in the mid-Miocene. 

 The overlying granite and its Mesozoic sedimentary veneer were domed 

 .y the injection and the top of the dome was largely destroyed by a 

 phreatic explosion. It was followed by the appearance of a little 

 liparitic tuft' erupted at a few points in the newly formed basin, but the 

 explosion itself was non-volcanic. 



According to Sekiya and Kikuchi, the great explosion of 18S8 at 

 Bandai-San was absolutely unaccompanied by the extrusion of lava.*^ 

 A priest living on the mountain survived the explosion. He reported 

 the vapors surrounding him to have been respirable, and the Japanese 

 geologists conclude from all available data that the catastrophe was a 

 steam explosion. There were no signs that juvenile gases formed an 

 important part of the volatile mixture. This " eruption " of Bandai- 

 San seems, therefore, to be an excellent example of a phreatic explo- 

 sion on a true volcanic cone. 



Phreatic eruption means steam-explosion without magmatic extru- 

 sion. Kilauea represents magmatic extrusion without steam-explosion. 

 Between these two extremes of terrestrial activity stands the type 

 representing the vast majority of active and extinct central eruptions. 

 In the non-volcanic or pseudo-volcanic activity of Bandai-San in 1888, 

 as in a Kilauea or a Vesuvius, true igneous injection is a pre-requisite. 

 The gases given off at Kilauea form a nearly pure juvenile mixture 

 with characteristic high temperature. The gases given oft" at Vesu- 

 vius form a mixture of juvenile, resurgent, and vadose volatile matter. 

 A type of the resurgent gas is the carbon dioxide set free in the de- 

 monstrable assimilation of Mesozoic limestone and dolomite in the 

 Vesuvian lava column. The gas and vapor given oif at Bandai-San in 

 1888 was apparently almost purely vadose or meteoric in origin. 



True volcanoes of the central-eruption type must vary enormously 

 in the relative and absolute proportions of juvenile, resurgent, and 

 vadose fluids composing their emanations. As the resurgent and 

 vadose fluids are volatilized, heat is lost and the viscosity of the 

 lava column rises. Assimilation of foreign rock in depth must lower 

 the temperature, and in the end, increase the viscosity and also the 

 average violence of explosions. In addition, magmatic dilTerentiation 

 generally brings the more silicious and more viscous pole to the ujiper 

 part of the lava column, and aids in the preparation of explosive 

 conditions. 



For these and other reasons, volcanoes of the central-eruption tyi)e 



*' S. Sekiya and J. Kikuchi, Jour. Coll. Science, Tokio, 1889, p. lOG. 



