94 Transactions. 



Of other minerals, hornblende is sometimes found, but is not 

 very frequent. Biotite is still more uncommon. Hypersthene 

 is by far the most usual of all ferro-magnesian minerals, especi- 

 ally in the southern portion of the district, though further north 

 its place is taken by hornblende in some measure. Augite is un- 

 common. The pumice offers no special peculiarities, for it is 

 merely vesicular scoria of the rhyolites. 



Few analyses of the rhyolites have been published. Hoch- 

 stetter* quotes some analyses of hot- spring deposits near Ro- 

 torua. Some of these appear to be silicified rhyolites. Mac- 

 laurin and Pondf give analyses of pumice. The percentages of 

 lime and magnesia are somewhat higher than is usual in this 

 type of rock. Determinations of silica are given in " Rocks 

 of Cape Colville Peninsula.''^ The percentage is rather more 

 than 70. 



There appears to be no record of rhyolites occurring any- 

 where to the north of Cape Colville, except in the Great Barrier 

 Island. The only example known to me is a dyke penetrating 

 the Manukau breccias at Karekare : it resembles those from 

 the Ongaruhe Valley. 



Trachytes : The only example of this group of rocks that 

 I have had was taken from one of the small hills near the Kai- 

 para. It is composed almost entirely of feldspar microlites, 

 but there is also a little biotite. 



Andesites : These rocks have a wider occurrence than the 

 rhyolites, and differ among themselves more in mineralogical 

 composition, but less in structure. 



Dacites have a considerable distribution in the Cape Colville 

 area, and many of them are coarsely porphyritic. Sollas has 

 described them under several names. Hornblende, pyroxene, 

 and hypersthene dacites all occur. The last are least frequent. 

 The minerals occasionally occur together, though hypersthene 

 and hornblende are not associated in more than two or three 

 specimens of dacites. 



Outside of the Cape Colville area dacites have not been re- 

 corded, so far as I know. I have, however, had specimens of 

 hornblende-dacite from the Hen and Chickens Islands, and in 

 the main volcanic area Tauhara is formed of a hornblende- 

 hypersthene-dacite. The hornblende has a peculiar reddish 

 colour. 



Of other andesites there is a great variety. The Cape Col- 

 ville Peninsula has numerous representatives of almost every 



* Eochstetter, " NVw Zealand," p. ».V>. 



t Pond and Maclaurin, Trans. N.Z. Inst., vol. xxxii, p. 233 et seq, 



X Vol. ii, pp. 303, 304. 



