384 REACTION OF THE INTERIOR OF THE EARTH 



gist. The trachytic mountain Orohena is said to be as 

 high as Etna. Tahiti has, therefore, next to the active 

 group of the Sandwich islands, the loftiest eruptive 

 rock in the whole of the great oceanic domain lying 

 between 1 the continents of America and Asia. A fel- 

 spathic rock of the small islands of Borabora and Mau- 

 rua in the vicinity of Tahiti, which later travellers 

 have termed syenite, and which Ellis in his " Polynesian 

 Kesearches" designates as a granitic aggregate of felspar 

 and quartz, is deserving of a much more exact minera- 

 logical examination ; as porous scoriaceous basalt breaks 

 forth in near proximity to it. Neither extinct craters 

 nor streams of lava are now found in the Society islands. 

 It is natural to ask, have the craters on the mountain- 

 summits been destroyed? or are the now riven and 

 altered remains of the volcanic frameworks the ruins of 

 closed domes ? or may we suppose that here, as has pro- 

 bably been the case at many other points of the up- 

 heaved sea bottom, basalt and trachyte beds have been 

 poured forth directly from earth-fissures? Extremes of 

 great tenacity or of great fluidity in the substances 

 which issued forth, as well as differences of narrowness 

 or breadth in the fissures through which they have 

 issued, modify the shape assumed by the volcanic rocks 

 formed by them; and where friction produces trituration 

 into small fragments and into what are then called 

 ashes, small, and most often transitory, cones of erup- 

 tion arise, which are not to be confounded with the 

 great terminal cones of ashes of permanent frameworks. 

 At only a short distance to the east of the Society 

 islands there follow the Low islands, or Paumotu. 

 These are mere coral-islands, with the remarkable ex- 



