384 



Transactions. — Geology. 



cal relations as mentioned above also point to the same con- 

 clusion. 



At the top of the hill along the strike of the dyke another 

 dark-coloured rock occurs of a far more basic character. The 

 geological relations of this rock to the tridymite-trachyte are, 

 however, exceedingly difficult to determine, as the whole sur- 

 face of the hill near the outcrop is covered with grass. 



The rock contains porphyritic crystals of plagioclase 

 twinned on the albite law, generally with idiomorphic out- 

 lines. A few crystals of sanidine were also seen in section. 



Augite is abundant, brown in colour, and displaying no 

 dichroism. Polysynthetic twinning parallel to the orthopina- 

 coid exists in the centre of some of the crystals, the more 

 peripheral portions being untwinned. No olivine w^as observed 

 in section. The ground-mass is glassy, with feldspar and augite 

 microlites, and abundant grains of magnetite. 



An analysis gave the following percentages : — 



SiOa ... ... ... ... 49-49 



Al,03 



Fe^Os 



CaO 



MgO 



KoO 



NaoO 



1805 

 12-94 

 9-81 

 3-24 

 1-30 

 604 



Both mineralogically and chemically this rock shows great 

 resemblance to typical basalts, and should be classed with 

 this species. 



Sections w^ere also made of the dyke that penetrates the 

 rock at B. Feldspar of a plagioclastic variety is abundant, 

 but no sanidine was observed. Augite is present in idiomor- 

 phic crystals of a brown colour, and is not dichroic. Apatite 

 is particularly abundant, and grains of magnetite are of fre- 

 quent occurrence. 



SiO.. ... ... ... ... 50-82 



Al.,0, 

 Fe.,0, 

 Cab 

 MgO 



14-79 



7-99 



11-10 



4-12 



The alkalies were not determined. The rock is undoubtedly 

 one of the ordinary augite- andesites of the Lyttelton system, 

 and bears no relation to the previously-mentioned rocks. 



Another rock outcropping close to the tridymite-trachyte 

 was also examined. Its geological relations could not be made 

 out, as all the surrounding slopes are covered with grass. 



Under the microscope numerous large porphyritic crystals 

 of plagioclase can be seen, and a few of sanidine, but the two 

 do not occur in contact. 



