] 16 Transactions. 



polarizing base. Another slide from the centre of one of the larger pillows 

 possessed similar characteristics in parts, but augite in bladed ragged forms 

 occupied the same relative position in regard to the feldspars that the 

 spicular growth did in the other sections. The last two varieties of the 

 rock can be paralleled exactly in sections cut from the intrusive rock in 

 Oarnaru Creek, west of Hutchinson Quarry. This rock is found to vary in 

 different parts of its mass, however, and a specimen taken from the lower 

 quarry was holocrystalline, and the augite enclosed the feldspars ophitically. 

 The rock shows another variation, in that larger feldspars and olivines are 

 embedded in a mass of granular augite and smaller feldspars. This specimen 

 is much coarser in texture than the other varieties. This granulitic variety 

 of the dolerite is probably due to movement towards the end of the process 

 of consolidation, while the ophitic type indicates that the cooling took place 

 under quiescent conditions. 



Pillow-lavas invariably belong to the group of intermediate or basic 

 igneous rocks. Among the Palaeozoic rocks well-developed pillow-lavas are 

 of frequent occurrence in Great Britain and are known as " spilites." 

 These rocks are characterized by their richness in soda and poverty in 

 potash, and mineralogically by the abundance of a soda-feldspar. Albite 

 is the principal constituent, next in importance is augite, and olivine 

 occasionally occurs. There is frequently a glassy base, occasionally the 

 feldspars are microporphyritic, and often those of the ground-mass have 

 pointed or acieular forms. Sometimes they consist almost wholly of 

 feldspar laths with a rluidal arrangement. In some types the augite may 

 occur as irregular masses enclosing the ends of feldspar rods, producing 

 a subophitic structure. Diabases, representing the intrusive magma, 

 invariably occur with these rocks. Flett (1911, p. 246) considers that 

 the characteristic feldspar of these rocks, albite, is due to the action of 

 pneumatolytic emanations containing water with soda and silica in solution 

 upon the basic feldspars soon after the rocks had solidified. Further, he 

 states that the spilitic suite of rocks are essentially rocks of districts that 

 have undergone a long-continued and gentle subsidence. 



The Tertiary pillow-lavas at Oarnaru contain no albite, and this is 

 confirmed by chemical analysis, which shows that the present rocks are 

 remarkably poor in soda, but, like the spilites, remarkable for their 

 poverty in potash. Their structure and mineralogical composition can be 

 paralleled in the spilites, except, of course, for the absence of albite. The 

 rocks were erupted under submarine conditions, but the water was shallow. 

 The association with the intrusive olivine dolerite is analagous to the 

 association of variolitic pillow-lavas with diabases in Anglesey and Cornwall. 

 Yet the poverty of the rock in soda precludes its classification as a spilite. 



VIII. Conclusion. 



1. There are three horizons of igneous rocks- — the Waiareka tuffs, 

 Kakanui breccia, and the upper pillow-lava — although the latter two were 

 almost contemporaneous, the breccia being the earlier. 



2. The stratigraphical unconformities and "limestone conglomerate*' 

 may be explained on the assumption that volcanic islands were rapidly 

 formed and rapidly destroyed. 



3. The unconformities introduced into the sequence by former observers 

 are merely local, and of no significance in classification. 



4. The Oarnaru series is otherwise conformable throughout. 



