• J. A. Thomson. — Igneous Intrusions of Mount Tapuaenuka. 315 



characters. The other two rocks are granitic in texture and structure, 

 but in each case the amount of quartz is rather low for a granite. One of 

 the two consists predominatingly of microperthite, with smaller amoimts 

 of plagioclase, hornblende, biotite, and accessory zircon, and might be termed 

 a quartz-syenite. The other contains augite in addition to the above mine- 

 rals, and in it the microperthite does not predominate so much over the 

 plagioclase ; it is a hornblende-granite or a quartz-diorite. 



Affinities of the Rocks. 



It cannot be definitely asserted that the acid rocks belong to the same 

 rock-series as the more basic rocks, but it is probable that they do. The 

 dolerites are not normal lime-alkali rocks, differing in the presence of titan- 

 iferous augite and in the abundance of biotite. At the same time, they are 

 not definitely of alkaline affinity. Chemical analyses are necessary before 

 the true position of the rocks can be made out, and it was not considered 

 advisable to undertake these on river-gravels, the more particularly as a 

 much better series of rocks could probably be collected from the dykes 

 in situ. 



A very similar series of rocks has been described by Bell and Fraser from 

 the Hokitika Sheet, North Westland Quadrangle.* They are described 

 as " pyroxene-camptouite, hornblende-camptonite, hornblende-porphyrite, 

 pyroxene-porphyrite, diabase, augite-diorite, and olivine-basalt." A re- 

 examination of their microscopical sections suggests that the " camptonites" 

 are better termed spessartites. The " augite-diorite " is very similar to the 

 homblende-albite rock described above from the Dee gravels, but is more 

 easily deciphered ; it is an albitized hornblende-dolerite, passing in places 

 into a hornblendite. Bell and Fraser believe the dykes to be of early or 

 middle Tertiary age, because of the lithological similarity to the olivine- 

 basalt of Koiterangi Hill, which rests on a denuded surface of the (Tertiary) 

 coal-measures. This lithological similarity is not brought out in their 

 petrographical description, and, as they admit that the dykes have been 

 found only in pre-Tertiary rocks, considerable doubt must attach to their 

 conclusion as to the age. 



Dr. J. Henderson has kindly shown me sections of many similar rocks, 

 found for the most part as boulders in river-gravels near Reefton. They 

 include pilite-spessartites, and some of them approach the odinites in the 

 structure of the groimdmass (Dr. Henderson had already identified the rocks 

 as spessartites and odinites). It is perfectly possible, however, that true 

 camptonites also occur in Westland and west Nelson, since alkaline plutonic 

 rocks (ditroite) are known. 



The closest analogies with British rocks lie with the pilite-spessartites 

 of the central Highlands, which are associated with vogesites.f 



ACKNOWLEDGMENTS. 

 Messrs. C. A. Cotton and J. A. Bartrum have given much help in the 

 writing of this paper, including the preparation of the thin sections. Mr. 

 Bartrum has also assisted in the determination of the mineral composition, 

 and in the identification of the rocks. 



* Bull. No. 1 (n.s.), Geol. Surv. N.Z., pp. 82-84 ; 1906. 



t Flett, J. S. : The Geology of Sheet 55, Mem. Geol. Surv. Scotl. ; 1905. 



