TRUE VOLCANOES. 399 



and crowned by a regularly rounded ash-cone. There is here 

 no trace of any stream of lava ; yet on the conical moun- 

 tain of Apia (2576 feet), which is likewise on Upolu, as 

 well as on the Peak of Fao (3197 feet) we meet with fields 

 of scoriaceous lava (Malpais of the Spaniards), the surface 

 of which is as it were crimped, and often twisted like a 

 rope. The lava-fields of Apia contain narrow subterranean 

 cavities. 



Tahiti, in the centre of the Society's Islands, far more tra- 

 chytic than basaltic, exhibits, strictly speaking, only the ruins 

 of its former volcanic frame- work, and it is difficult to trace 

 the original form of the volcano in those enormous masses 

 looking like ramparts and chevaux-de-frise, with perpendicu- 

 lar precipices of several thousand feet in depth. Of its two 

 highest summits, Aorai and Orohena, the former was first 

 ascended and investigated by that profound geologist Dana. 1 

 The trachytic mountain, Orohena, is said to equal Etna in 

 height. Thus, next to the active group of the Sandwich 

 Islands, Tahiti contains the highest rock of eruption in the 

 whole range of the Ocean between the Continents of Ame- 

 rica and Asia. There is a felspathic rock on the small 

 islands of Borabora and Maurua, near Tahiti, designated by 

 late travellers with the name of syenite, and by Ellis in his 

 Polynesian researches described as a granitic aggregate of 

 felspar and quartz, which, on account of the breaking out 

 of porous, scoriaceous basalt in the immediate neighbour- 

 hood, merits a much more complete mineralogical investiga- 

 tion. Extinct craters and lava-streams are not now to be 

 met with on the Society Islands. The question occurs, 

 are the craters on the mountain tops destroyed, or did the 

 high and ancient structures, now riven and transformed, con- 

 tinue closed at the top like a dome, while the veins of basalt 

 and trachyte poured immediately forth from fissures in the 

 earth, as has probably been the case at many other points of 

 the sea's bottom? Extremes of great viscidity or great 

 fluidity in the matter poured out, as well as the varying 

 width, or narrowness of the fissures through which the effu- 

 sion takes place, modify the shapes of the self-forming vol- 



1 Leop. von Buch, p. 383 ; Darwin, Vole. hi. p. 25 ; Darwin, Coral 

 Reefs, p. 138 ; Dana, pp. 286305 and 364. 



