1-22 



Transactions. 



consisting of feldspar, magnetite, and augite, without glass. Flow structure 

 is apparent in some localities and absent in others. 



Phenocrysts. — Phigioclase phenocrvsts vary in size up to 1-5 mm., and 

 are abundant. Polysynthetic albite twinning is universal. The extinction- 

 angle of adjacent lamelLnp in sections perpendicular to 010 is 30°. The feld- 

 spar is therefore labradorite. 



Augite phenocrvsts are large, being as much as 7 mm. or 8 mm. in length 

 and 2-8 mm. in thickness. The colour is grey, and the maximum extinction- 

 angle 41°. The crystals are idiomorphic, and some show twinning, with 

 the orthopinacoid as composition plane. 



Magnetite occurs in crystals and rounded grains up to 04 mm. in dia- 

 meter. 



Olivine occurs in rounded crystals up to 0-5 mm., and, more rarely, 

 larger. It is pale in colour. The olivine crystals are often replaced by 

 serpentine pseudomorphs. 



Groundmass. — The groundmass consists chiefly of irregularly arranged 

 small needles and laths of labradorite and numerous irregular grains of grey 

 augite and magnetite. At some places feldspar is very abundant, with 

 flow structure, and at others no flow structure is to be seen, while magnetite 

 and augite are relatively more abundant. 



Chemical Composition. — For comparison the analysis of a dolerite* from 

 Dyer's Pass, Canterbury, is quoted : — 



Bamlt No. 2. 



This basalt is not found in situ, but only as spheroidal cores in a deep 

 clay. It has a cubical fracture. The minerals are similar to those in No. 1. 

 and occur in the same proportions. The groundmass is similar to that of 

 No. 1, the only difference being in the phenocrysts. The feldspar pheno- 

 crysts are much larger than in No. 1, and the olivine is nearly all decom- 

 posed. 



Basalt No. 3. 



This rock, which occurs in a dyke, is very hard and dense in appearance. 

 Olivine crystals are seen in abundance. 



Microscopic Characters. — In sections the rock is holocrystalline and 

 porphyritic. The groundmass is much more coarsely crystalline than that 

 of the lava basalts. 



* R. Speight, Trans. N.Z. Inst., vol. xxvi, p. 409. 



f MgO appears low to be associated with such a low amouiit of SiOo- 



