272 Transactions. — Geology. 



ore. He considers that this series rests unconforrnably on the 

 ' Annelid beds,' which latter he thinks form the upper part of 

 the Maitai series. 



Last summer one of us examined these rocks in the field, 

 and agreed with former observers that they were volcanic ashes 

 and lava streams contemporaneous with the sandstones and mud- 

 stones among which they are found. In places, near their 

 margins, considerable decomposition has gone on, and the green 

 rock is penetrated by bright red siliceous veins, coloured by ferric 

 oxide, forming the so-called red cherts and jasperoid rocks. 

 Calcite veins are often associated with these jasperoid portions, 

 giving further evidence of decomposition, and in places the 

 calcite forms segregation masses from 20 to 25 feet thick, which 

 have been called marble, or even limestone. However, their 

 irregular and lenticular shape, as well as their intimate junction 

 with the greenstone along the line of junction, as well as the 

 mineralogical character of the calcite, which does not resemble 

 limestone but vein calcite, all go to prove their real nature. 

 Small quantities of copper and manganese have been found 

 associated with these rocks ; and rocks from Okuku, said to be 

 lithologically similar, called ferruginous cherts, contain a small 

 percentage of gold.* 



The undecomposed igneous rock is compact, of very fine 

 grain and of a darkish-green colour, too fine-graiued to call a 

 diabase and too light-coloured to call melaphyre ; it answers 

 better to the old name of aphanite. The hardness is about 4'5 

 or 5, and the specific gravity varies between 2-96 and 3-05. On 

 the points it has a greasy lustre like serpentiue, but not so well 

 marked. 



With an inch objective and ordinary light the rock looks 

 granular, numerous small colourless or pale olive-brown, much 

 cracked, crystalline grains in a translucent mesh-work of ground- 

 mass which is milky or cloudy with occasional whitish patches 

 of leucoxene, and rarely black specks of iron ore. With reflected 

 light the ground-mass is very pale-green, and the crystallized 

 mineral is colourless. The iron ores are pyrites and a brownish- 

 black oxide, no doubt ilmenite ; the pyrites is rare. With crossed 

 nicols the crystallized mineral shows brilliant polarization colours. 

 Crystalline faces are rare, the mineral being generally in coarse 

 granules, which look as if they had been broken apart ; but on 

 revolving the stage, it is seen that the grains have generally 

 independent orientation, although sometimes two grains sepa- 

 rated by ground-mass extinguish together. But there are no 

 large single crystalline masses. These grains are mostly of a 

 pale yellowish-green colour, but some are darker and of a 

 brownish-green. None are pleochroic. Occasionally pale-green 



* " Rep. Geol. Expl." 1879-80, p. 105. 



