424 Transactions 



sections- — a very distinct type, perhaps outside the series- — there are 

 abundant idiomorphic crystals of the green hornblende, and a large pro- 

 portion of cryptoperthite, whilst quartz is abundant. 



It is evident from the foregoing description that it is difficult to classify 

 these rocks : there is an element of doubt about the origin of much or 

 most of the hornblende ; but, from several considerations, it appears that 

 they may provisionally be classed as quartz-hornblende gabbros. 



Finally, the sections cut represent normal dioritic rocks, unlike the 

 former in that the plagioclase is now predominant and the structure is 

 hypidiom orphic. The hornblende in unaltered rocks is a deep-bluish-green 

 to golden -brown variety. Quartz and augite are absent. Sections of 

 another diorite show that it has been subjected to pressure which has 

 caused partial granulation. The hornblende has been a strongly schiller- 

 ized deep-brown variety, but is now converted in great measure to an 

 opaque whitish-grey product. 



Trachyte, Miocene Breccia-conglomerate, Wairau Creek, Milford, 



Auckland. 



Mr. C. E. Fox, in his admirable paper on the volcanic beds of the 

 Waitemata series, Auckland, noted in the coarser fragmental beds that 

 " besides the more basic fragments there others present the appearance of 

 true trachytes, pale-grey in colour, with a specific gravity of 2-54.'"* From 

 his statement in connection with the fragments, " On the whole, then, they 

 may be taken as typical andesites, while a small percentage are basalts 

 without olivine, "f one judges, perhaps wrongly, that he regards these 

 trachytic rocks as a phase of the andesites. 



There is an interesting assortment of andesites, some with a little 

 olivine, J in the Wairau Creek volcanic breccia-conglomerates at Takapuna, 

 but the trachyte that has been found there by the writer shows but little 

 mineralogical resemblance to any of these. It was discovered in several 

 boulders up to 10 in. in diameter, all most perfectly rounded, and con- 

 trasting sharply with the general subangular and rough material of the bed 

 in which they were found. 



Microscopically the trachyte consists of idiomorphic phenocrysts of clear 

 orthoclase, slightly subordinate plagioclase, and a little pale augite, in a 

 dense finely trachytic groundmass showing perfect fluxional arrangement. 

 The feldspar laths enclose between them very fine granules of augite and 

 magnetite. The proportion of plagioclase laths to those of alkali feldspar 

 is somewhat higher than in typical trachytes, a feature to be noticed in 

 most of our New Zealand trachytes. 



Trachytes are rare in the North Island of New Zealand, the only 

 occurrence in situ being at Pukekaroro, near Maungaturoto, and the pre- 

 sence of the rock in the Wairau Creek volcanic fragmental bed is therefore 

 interesting. 



* C. E. Fox. The Volcanic Beds of the Waitemata Series, Trans. N.Z. Inst., vol. 34, 

 1902, p. 426. 



t C. E. Fox, loc. cit. 



X See also E. K. Mtjlgan, On the Volcanic Grits and Ash-beds in the Waitemata 

 Series, Trans. N.Z. Inst., vol. 34, 1902, pp. 420, 423, 424, &c. 



