THE ACTIVE VOLCANOES OF NEW ZEALAND 713 



rhyolite, but about 175 feet of dark, reddish-brown scoriae con- 

 sisting of ashes, lapilli, and bowlders of vesicular and ropy lava 

 lies along the brink of the great chasm through Mount Tarawera. 

 The lava, which was the last to fall on the mountain, except some 

 material from the basin of Rotomahana, welled up beneath the 

 chasm and was caught in the big explosion. It was blown to 

 fragments and thin layers of the fine material are mixed with the 

 lighter colored tuff from the rhyolites around Rotomahana. It 

 formed irregular masses of scoriaceous and ropy lava up to two 

 feet in length, while it quite frequently formed spherical and oval 

 bombs (Fig. 13). These bombs occur in great numbers around the 

 foot of the mountain. The most peculiar are those with a core of 

 rhyolite and an enveloping coat of andesite or basalt. They owe 

 their origin to the fact that fragments of rhyolite were engulfed 

 in the more basic lava, and when the explosion occurred these were 

 hurled into the atmosphere with a rotary motion so that the viscous 

 molten material became well wrapped around the core of solid rock. 

 As a rule, this core is not exposed until the bomb is broken open. 

 They all show the bread-crust structure well developed owing to 

 the shrinking of the cooling, molten coat around the solid interior. 

 In the specimens examined there is a sharp line of contact between 

 the two rocks and there is no evidence of fusion of the rhyolite. 



An examination of thin sections of the more basic rock showed 

 in one case much dark, grayish-brown, vesicular glass containing 

 numerous little laths of feldspar, a little augite, and a few small 

 phenocrysts of enstatite. In another specimen the same minerals 

 were found, with the exception of augite. In one small bomb the 

 feldspars were identified from their extinction angles as anorthite 

 and bytownite, and this same specimen contained traces of quartz, 

 possibly due to absorption of some of the acid rhyolitic material 

 before ejection. It was carefully examined for nephelite, owing to 

 the reported occurrence of nephelite in the Auckland lavas, but it was 

 found to be optically positive and to lack any sign of cleavage. Very 

 small crystals of augite were present. The rock is a quartz basalt. 



Dr. Marshall mentions hypersthene-augite-andesite in the 

 bombs from Mount Tarawera, 1 but no hypersthene has been found 



1 Marshall, op. cit., p. 102. 



