Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/84

 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 without upturning their edges in the least, and covered them with accumulations 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 formation of the synclinal and the eruptions of the volcano took place subsequent to the deposition of the yellow sandstone ; but there is no evidence to show whether the synclinal movement was before or after the eruption. Still, as we cannot suppose that subsidence took place while the pressure below was so great that it found vent in breaking through the overlying 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 synclinal of so small an extent (only 10 miles) could be caused by subsidence into a cavity at a considerable depth, the conclusion ap-