The Taravjera Outbreak. 119 



The use of these qualifying words in relation to the scale 

 and to the origin of the eruption decides me that Dr. Hector, 

 when he used the phrase "a purely hydro thermal pheno- 

 menon," meant to assert that Tarawera is a geyser, and not 

 a volcano. 



Now, what is the difference between a volcano and a 

 geyser ? They are alike as to their causes. Both of these 

 forms of activity are due to the same agent. Each is a 

 natural steam-engine. Each requires to have water in its 

 chambers before it can operate, and without water can no 

 more act than can a locomotive with a dry boiler. But if 

 they agree in their motive-power, they differ greatly in the 

 mode in which they eject solid matters ; and they differ 

 even more widely in the nature of the material ejected. 



Geysers emit usually only water and steam, but occasion- 

 ally they eject stones. Such stones are, however, always of 

 superficial origin; they are rocks which have been torn out 

 of the vent by the steam blast ; they are derived from its 

 throat, and not from its stomach. The ejectamenta of a vol- 

 cano are, on the contrary, drawn mainly from its stomach — 

 that is, they are principally of plutonic origin. Only a small 

 proportion of the output is derived from the throat, and this 

 part is ejected chiefly at the beginning of the outbreak, 

 while the fissure is forming, or when the plug is being blown 

 out. Therefore, the first difference between a geyser and a 

 volcano is that the former ejects only steam and the materials 

 of the neck, whereas volcanoes vomit the contents of their 

 deep-seated reservoirs as well as the contents of their 

 vents. 



The second difference between them is a direct consequence 

 of the first. 



While the solid ejecta of geysers is nnfused country rock, 

 and never consists of lava, scoria, or ash, volcanoes eject all 

 these materials, and the country rock from the neck is, as 

 often as not, more or less fused. 



Consequently, we should be able to decide as to whether 

 Tarawera is a volcano or whether it is a mere geyser, as Dr. 

 Hector contends, by an examination of its ejectamenta. 



Both Dr. Hector and Mr. Percy Smith describe the 

 materials erupted by Tarawera, and they tell us that 1800 

 square miles of country have been buried more or less deeply 

 ■under sand, ash, and tuff, mingled with fragments of old 

 trachytic lava, the latter being the country rock drilled out 

 of the vent. 



