14 



PKOCEE DINGS OF THE GEOLOGICAL SOCIETY. [NoV. 25, 



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through by the basaltic cone of Nga 

 Tutura. This hill is about 600 

 feet high, with a gently rounded 

 summit, showing, from the land 

 side, no sign of its volcanic origin. 

 On the sea-shore, however, at low 

 tide, one is able to climb out on 

 the fallen masses of basalt, and 

 examine its internal structure. In 

 this way it is seen that the volcano 

 has burst through the rocks with- 

 out upturning their edges in the 

 least, and covered them with ac- 

 cumulations of tuff and lava. The 

 lower part is filled with ashes and 

 scoriae, enclosing large irregular 

 blocks of the yellow sandstone, and 

 dipping (roughly) N.W. and S.E. 

 from the centre. Above, the hill 

 is chiefly composed of lava streams 

 of basalt, containing olivine, with 

 but little tuff, which fact and the 

 denuded outline of the cone seem 

 to show that the eruptions were 

 submarine. 



From the foregoing description 

 it is evident that both the forma- 

 tion of the synclinal and the erup- 

 tions of the volcano took place 

 subsequent to the deposition of the 

 yellow sandstone ; but there is no 

 evidence to show whether the syn- 

 clinal movement was before or 

 after the eruption. Still, as we 

 cannot suppose that subsidence 

 took place while the pressure be- 

 low was so great that it found 

 vent in breaking through the over- 

 lying rocks, it seems to follow that 

 they must have sunk down after 

 the volcano had exhausted its 

 energy, and that at the same time 

 the portion between the volcano 

 and the fault south-east of it fell 

 through lower than the rest ; and, 

 as I cannot imagine that a syn- 

 clinal of so small an extent (only 

 10 miles) could be caused by sub- 

 sidence into a cavity at a consi- 

 derable depth, the conclusion ap- 



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