272 Transactions. 



tMrd a light-coloured fine-grained rock with few phenocrysts. This latter 

 rock contains nests of tridymite, and therefore for the sake of distinction 

 will be designated the " tridymite-rhyolite." 



(«.) SpheruJitic Rhyolites. 



Macroscopically the spherulitic rhyolites show numerous spherulites, 

 many grains of quartz, and occasional small plates of biotite. A little 

 secondary opal is present in some specimens, and may show good " fire." 

 Under the microscope quartz appears in large grains, with many gas en- 

 closures. Small crystals of plagioclase and biotite may also be present. 

 The greater part of every section, however, is seen to consist of spherulites. 

 The exact nature of the spherulites in Hauraki rhyolites has been discussed 

 by Rutley (5, pp. 451 et seq.) and Sol las (11, vol. 1, pp. 120-22). Rutley 

 regards them as being composed of devitrified glass, whilst Sollas, beyond 

 adducing evidence opposed to the devitrification theory, makes no very 

 definite pronouncement.* For the present it may be assumed that the 

 spherulites approach orthoclase in composition, and were formed during 

 the final consolidation of the rhyolites. 



In a section of rhyolite from the hills about a mile and a half east of 

 Waihi a quartz-grain is seen to form part of a spherulite boundary. Hence 

 in this case the spherulite is obviously of later formation than the quartz. 

 In the same section a phenomenon which may be termed spherulite within 

 spherulite occurs. 



At Waihi Beach a beautifully spherulitic rock is traversed by auriferous- 

 quartz veins. Sections show that the rock is more or less silicified, much 

 secondary quartz mosaic being present. Feldspar (probably orthoclase) 

 appears in imperfect crystals, one of which is imbedded in a spherulite. 



A small vein traversing one of the sections from Waihi Beach consists 

 of a fine quartz mosaic cementing and in part replacing fragments of coun- 

 try. . It may from this be inferred that the auriferous veins of this locality, 

 like those at Waihi, are largely replacement veins following fracture planes 

 or zones. 



The rhyolites referred to above are more fully described by Sollas (11, 

 vol. 2, pp.' 35, 112). 



(b.) Wilsonite. 



The peculiar rhyolitic rock commonly known as " wilsonite " has been 

 described by Sollas '(11, vol. 1, pp. 123, 124 ; vol. 2, pp. 46, 138). The same 

 rock is mentioned by Park (3, p. 88) and McKay (4, p. 67), and has probably 

 been described by Rutley (5, pp. 460, 461, No. H^s). 



The freshest obtainable specimens of wilsonite show pinkish or purplish 

 surfaces flecked with numerous black streaks. The fractured rock has a 

 somewhat vitreous lustre. When affected by surface weathering the pur- 

 plish portion of the rock become nearly white, and the dark portions tend 

 to approach a light-grey colour. Highly weathered rock is of a nearly 

 uniform almost white colour. In places wilsonite tends to pass into a tuff, 

 owing, it is believed, to brecciation of surface portions that solidified before 

 the cessation of flow. Rounded fragments of andesite, usually from ^ in. 

 to 1 in. in diameter, are very common as inclusions in the wilsonite. These 

 are most noticeable in the highly weathered rock, which also is usually 

 the most tufaceous-looking. 



* Sollas stated (11, vol. 1, pp. 122, 124) that he intended to deal more fully with the 

 questions of structure, origin, &c., in a final report. This, apjian-ntly, was not written. 



