The Tarawera Outbreak. 121 



believes to be its absolute depth or what he would call deep 

 or shallow. This phrase of his, then, as descriptive of 

 position, is of vague and uncertain meaning, for depth is 

 relative and ideas differ. The depth that would be great 

 for a shaft would be shallow for a volcano, and, compared 

 with the globe-bulk, the greatest sea-depth is superficial. 

 Now, it is impossible for any one to prove, in absolute 

 measure, what the position of the focus is ; but I think I 

 have shown that its depth must be that of a reservoir capable 

 of holding for a long period molten diorite mixed with water 

 at a white heat. Therefore, it seems to me that Dr. Hector 

 can establish his contention of a shallow origin for Tarawera 

 only by showing us that diorite can be elaborated under 

 a crust which, judging by an analogy, is too thin to aflford 

 it either great pressure or a hermetic cover, and within a 

 furnace the wall of which is not thick enough to retain the 

 heat required to fuse its materials.*!* 



Taking into consideration the plutonic nature of the 

 ejected materials, therefore, I am unable to accept Dr. 

 Hector's conclusion that Tarawera is a geyser eruption. 



But when we have decided in our own minds that it is a 

 volcano and not a geyser, there is still room left for further 

 inquiry into its nature, as volcanoes are of several types, 

 distinguished from each other by important differences. 

 For instance, Dr. Hector does not notice in his report what 

 appears to me to constitute the essence of the nature of this 

 eruption — namely, that it is a fissure eruption and not a 

 crater eruption. Certainly, he points to the fissure, and 

 terms it " the most remarkable and characteristic feature of 

 the late eruption;" but while he recognises the greatness of 

 the feature as a part of the landscape, he does not say a 

 word to indicate that he recognises the real importance of 

 the fissure to rest upon the fact that it supplies the key 

 which explains a great enigma in the mechanics of Tarawera, 

 which otherwise would puzzle us. 



This enigma may be stated as follows : — The Tarawera 

 vent was charged with lava ; the vent was so low in posi- 

 tion that it was level with the lake ; the lava was charged 

 with an abundance of interstitial steam, and yet no lava 

 was erupted. Why, then, with all these favourable condi- 

 tions present, was there no emission ? 



f Judd estimates that the Granites have consolidated under the pressure 

 of depths varying between 30,000 feet and 80,000 feet. Volcanoes, p. 253. 



