120 The Tarawera Outbreak. 



Dr. Hector also presents to us an analysis of the dust from 

 Tarawera, and it is constituted as follows : — 



from Tarawera. 





Obsidian. 



Andesitie 

 Lava. 



Diorite or 



Syenite. 



(Dr. Hector's 



report. ) 









Si. 





59-37 



60 to 80 



59-75 



54 to 59 



59-83 



Iron oxides 





1018 



3 to 7 



7-57 



10 to 14 



701 



Al. 





17-96 



18 to 19 



17-25 



16 to 18 



16-85 



Manganese 





traces 



— 



— 



— 



— 



Ca. 





5-98 



11 



6 



6 to 7-5 



4.43 



Mag. 





1-19 



0-6 



1-30 



6 



2-61 



Phos. acid 





traces 



— 



— 



— 



— 



Water 





2-21 



— 



1-0 



— 



1-29 



Soluble salts 



... 



traces 



— 



— 



— 



— 



Organic matter 



•99 



— 



— 



— 



— 



Alkalies 





212 



9-2 



710 



— 



9-18 



Now, we are sufficiently well-acquainted with the nature 

 of volcanoes to be able to say that an obsidian ash of the 

 composition given above has been derived from that variety 

 of lava which has been named andesite, and that andesitie 

 lava in its turn has been derived from a reservoir of molten 

 diorite. I therefore give the elements of these rocks side 

 by side with those of the ash, so that you may see at a 

 glance their elementary kinship.* These three forms of the 

 same substance (obsidian, andesite, and diorite) result 

 entirely from their elements cooling under different con- 

 ditions ; but their fundamental agreement is disclosed in 

 their identical composition. This volcanic dust consists 

 of a mass of broken glass bubbles and spicules, which 

 could not have been derived from the throat of a geyser, 

 for it has required for its elaboration conditions available 

 only at great depths. If Tarawera has ejected an ash which 

 must have been elaborated at great depths, then we must 

 admit that it must have a deep-seated origin; but we have 

 already noted that Dr. Hector denies this. He declares that 

 its origin is not deep-seated/ but he neither tells us what he 



* The relationship of the various plutonic rocks to the volcanic forms is 

 stated thus by Professor Judd : — 



Granitic Forms. Lava Forms. Glassy Form. 



Acid ... Granite ... Ehyolite ... Obsidian 



i Syenite ... Trachyte ... Do. 



Diorite ... Andesite ... Do. 



Miascite ... Phonolite ... Do. 



Basic ... Gabbro ... Basalt ... Tachylyte 



