Marshall. — Geology of North Island. 95 



type, though I do not know of descriptions of any mica-andesite. 

 Hornblende-andesites are less usual than hypersthene-bearing 

 varieties, and augite-andesites are not very common. Two or 

 more of these minerals may occur together. The structures, 

 too, are many. Besides the ordinary structures of andesitic 

 rocks, Sollas has described the micropoecillitic, in which quartz 

 forms grains of relatively large size, with highly irregular bound- 

 aries, and in the grains are included the constituents of the 

 groundmass. Spherulitic varieties are also described in some 

 number. 



Mount Egmont consists entirely, so far as my researches go, 

 of andesitic rocks. The usual type is a hornblende-augite- 

 andesite, in which the augite is a pale green. The hornblende 

 is sometimes completely resorbed, and an augite-andesite re- 

 sults. Occasionally a little olivine is found. This description 

 agrees with that of other workers, though Hutton first described 

 the Sugarloaf rocks as trachytes, and he has also mentioned a 

 hypersthene-augite-andesite from this locality. Sollas mentions 

 a little hypersthene in one type : I have found none in any of 

 my sections. Mr. R. Browne sent me some fine lamellar speci- 

 mens of haematite which were obtained from a tuff-bed on the 

 lower slopes of Mount Egmont. 



Ruapehu and its neighbours are entirely formed of hyper- 

 sthene-augite-andesite, so far as I know. Specimens have been 

 collected all over the east and south sides of the mountain, and 

 from the west and north sides collections have been made from 

 streams. The augite is pale brown, and the hypersthene is 

 strongly pleochroic. There is no hypersthene in the groundmass, 

 which is usually hyalopilitic, though sometimes pilotaxitic. A 

 little olivine is occasionally found. It is usually surrounded by 

 numerous hypersthene crystals. I have found no hornblende 

 in any of my numerous specimens, and no examples of phonolites, 

 basalts, or trachytes, mentioned by Park and Hill. In Thomas's 

 descriptions of the rocks of these mountains there is no men- 

 tion of hypersthene. This must be regarded as an oversight, 

 for the mineral occurs so invariably in my specimens that I 

 cannot fail to think that some, at any rate, of his must have 

 contained it. 



Hypersthene-andesites are recorded by Hutton from many 

 •other localities, and augite-andesites from many by Thomas. 

 The latter mentions this rock as the product of the eruption 

 of Tarawera in 1886. This statement has been confirmed by 

 Hutton and Rutley. The specimens I have gathered from 

 this volcano are hypersthene-augite-andesites again, but the 

 rock is very fine-grained, and identification of the minerals is 

 not easy, but there is no doubt that hypersthene occurs. 



