Thomas. — Volcanic Rocks of the Tempo District. 309 



Near the Karutau Stream, on the west of Lake Taupo, was 

 found a very black rock of resinous lustre, and showing only a 

 few small crystals to the naked eye. The microscope shows 

 that the larger crystals — the first separated in the cooling of the 

 rock — are principally plagioclases, though a few augites are 

 present. The plagioclases have very numerous inclusions of 

 glass, which is sometimes brownish, but usually greyish and 

 partly devitrified. The ground-mass consists of a grey glass, 

 containing numerous microliths and magnetite grains ; but here 

 and there patches of brownish glass, free from microliths, are to 

 be seen. It contains, also, numerous ledge-shaped sections of 

 small plagioclase, which show a flow- structure by their arrange- 

 ment. 



The curious peak of rock on the summit of Titiraupenga, 

 which forms so conspicuous a landmark when viewed from the 

 country north of Taupo, is composed of a rock which must be 

 considered as an augite-andesite. It is a rock of interesting 

 appearance, having a dark-grey ground-mass, in which are 

 embedded abundant greenish-black augite crystals, measuring up 

 to -Jrd of an inch in diameter, and more numerous felspars up to 

 |th of an inch in diameter. There is no olivine. The felspars 

 are chiefly plagioclases ; they are much fissured, and have fairly 

 abundant glass inclusions. The augites have very few inclusions, 

 their borders in some cases being sharply defined, but, as a rule, 

 they are bordered by a single row of crystalline augite grains, 

 which form, as it were, the outermost layer, or zone, of the 

 crystals. The ground-mass is distinctly micro-crystalline, being 

 composed of augite grains, felspar, and magnetite, with only a 

 small proportion of amorphous matter between. 



It should, perhaps, be mentioned that augite-andesites were 

 found by Hochstetter in the country lying further to the north- 

 east of Lake Taupo — as, for instance, at Kakepuku and Pirongia, 

 extinct volcanic mountains lying on the right and left banks 

 respectively of the Waipa. 



Rocks collected by Mr. Cussen at Maungakawa are also 

 augite-andesites. 



Rhyolites.- — The rhyolites occur in a greater variety of struc- 

 tural forms than perhaps any other species of rocks. Some of 

 the varieties are eminently glassy (forms of obsidian, pumice, 

 etc.). Another group — the Ehyolites proper of Zirkel, but 

 named by v. Eichthofen and others the Liparites — includes felsitic 

 and porphyritic varieties. Both these groups are represented 

 by numerous varieties in the Taupo District. A third group, 

 the Nevadites or granitic rhyolites of v. Eichthofen, appear to the 

 naked eye to consist mainly or entirely of crystals of quartz, 

 sanidine, etc., the ground-mass being present in small quantity. 

 This last group is not represented in the district, so far as I am 

 aware. 



