PROCEEDINGS OF SECTION C. 375 



Wanganui at Pipiriki, for the river and its tributaries drain the western 

 side of the mountains. Here again I found nothing but hypersthene 

 andesites, so I am forced to think that the other identifications are due 

 to mistakes of some kind. On Ruapehu, at 6.500ft. on the south-west, 

 a spur is composed of an exceedingly fissile light-grey andesite, with 

 distinct grains of olivine. Under the microscope these grains are found 

 to be surrounded by small crystals of hypersthene. This fact appears 

 to confirm Professor Sollas' statement that in some of the Thames ande- 

 sites hypersthene seems to replace olivine. Hutton has described 

 h}'persthene andesites from many of the other cones. It may, there- 

 fore, generally be stated that the cones that project from the rhyolite 

 plateau are composed of hypersthene andesites. It will be remembered, 

 on the other hand, that the rock expelled as banks from Mount Tara- 

 wera was described by Thomas as an augite andesite. My specimens 

 of this rock are highly vesicular fine-grained hypersthene augite ande- 

 sites. 



In the region of the large volcanoes some of the lava flows have 

 been completely eroded, especially in Tongariro, where the valleys are 

 sometimes 500ft. deep in the gently sloping lava flows. Into these valleys 

 recent lavas have flowed from Ngauruhoe. This seems to indicate a 

 period of relative quiescence of considerable duration. The formation 

 of the cone of Ngauruhoe, and of some craters in Tongariro, are the 

 result of recrudescence of activity. 



Outside of the main volcanic plateau there are isolated centres of 

 activity. Mount Egmont is one of these. The specimens given me 

 by Mr. R. Browne, from all parts of the mountain, are hornblende 

 augite andesites, with a hyalopilitic ground mass. The rock is quite 

 similar to that of the Sugar Loaves at New Plymouth. It has been 

 described by Hutton. The specimen of this type of rock described by 

 Hutton from Mount Ruapehu was probably wrongly labelled, for he 

 describes a hypersthene andesite from the Sugar Loaves which, from 

 the description, appears to be a Ruapehu rock, .^fter careful search 

 in these two localities for the rocks described, the author has come to 

 the conclusion that Hntton's labels were mixed. 



Two other isolated centres are at Pirongia and Karioi. The rocks, 

 in common with those of Ruapehu, and of other andesitic cones, have 

 been called trachydolerites My specimens are all dolerites, with 

 large grains of olivine and augite. Hochstetter mentioned the former 

 as a basaltic cone. 



From the basin of the Waikato, almost as far south as Drury, to 

 Auckland, in the north, there is a plateau of basalt. Its complete 

 continuity is interrupted a little south of Auckland ; but the numerous 

 cones on the Auckland isthmus supply a continuation. In its southern 

 extension the surface of the basalt is very much weathered, while the 

 surface of the lava flows from many of the Auckland cones is still 

 absolutely fresh. Notwithstanding this apparent difference in age, 

 the microscope shows that the rocks are essentially identical. In all 

 cases that I have tested, the rock powder gelatinises with acid, and the 



