310 Transactions. — Geology. 



Very numerous specimens of rhyolites were collected by Mr. 

 Cussen, more especially on the western side of the lake. I do 

 not propose to deal with all the varieties found, but merely to 

 mention some of the leading forms. 



A rhyolite from Whangainata Bay has a distinctly granitoid 

 appearance, showing numerous white crystals, with angular 

 granules of darker substance between. A little closer observa- 

 tion, however, suffices to show that the darker portions are 

 merely the ground-mass of a dark-grey glass in which the 

 crystals are embedded. The latter consist of cracked glassy 

 blebs of quartz and sanidines, with some plagioclase. The rock 

 contains, besides, a few small angites. The ground-mass is a 

 nearly pure glass, alternating with a glass densely crowded with 

 long slender pellucid microliths which do not polarise. The 

 alternation of bands of these two varieties of glass in wavy lines 

 makes evident the fluidal structure of the rock. 



Another rock from the same locality is grey in colour, the 

 macroscopic crystals are few in number ; they are mostly sani- 

 dines, with a smaller proportion of plagioclase. Quartz seems to 

 be absent, and the rock contains accessory crystals of dark- 

 brown hornblende and pale augite ; both are scarce. It may 

 be added that augite, in small amount and in small crystals, is a 

 very frequent mineral in the rhyolites of the neighbourhood of 

 Lake Taupo ; in other parts of the Taupo zone I have found it 

 much less frequently. The ground-mass in the rock is sphaero- 

 litic, consistiDg of granules, and greyish non-polarising granules, 

 arranged in small spherical masses, the fibres radiating from the 

 centre of the spheres. In this case the spha?rolites show no 

 tendency to separate from one another, and are not visible to the 

 naked eye. 



A grey rock from the same place contains a large number of 

 macroscopic crystals, mostly quartz and sanidine, with a con- 

 siderable proportion of sanidine; but both hornblende and augite 

 are present in very small amount. The ground-mass is micro- 

 felsitic, with a strong tendency to form axiolites in which the 

 fibrous material diverges, not from a point as in the sphauolites, 

 but from along a line or axis. It may be noted here, that, 

 whilst in the typical rhyolites the orthoclase felspars (sanidines) 

 predominate, plagioclase being present in much smaller propor- 

 tion, yet in some of the varieties of the Taupo volcanic zone the 

 proportion of plagioclase rises occasionally so as to equal that of 

 the sanidine, and, possibly, surpass it, suggesting the idea that 

 the rock is a dacite. Nevertheless, on the whole, the sanidine 

 exceeds the plagioclase in amount in these r< t-ks. 



A rhyolite from Western Bay has a ground-mass which is a 

 mixture of wavy fluidal bands and streaks of lilac and bluish, 

 black stony substance. The crystals contained in the ground- 

 mass are mainly sanidines, quartz is absent, and plagioclase and 



