168 



Transactions. 



Groundmass. — Feldspar laths (0-24 mm. X 0-88 mm.) are of labradorite 

 twinned once or twice. They show a flow-structure. A few grains of 

 olivine and magnetite are also present. 



The following is an analysis of the basalt : — 



Basalt No. 2 occurs on the north-east of the Chain Hills, where it rests 

 on the schist. In the water-race to the west of Abbott's Hill Koad the 

 rock, which is in the form of hexagonal columns, has at its base a bed of 

 agglomerate. Owing to the absence of outcrops of basalts Nos. 1 and 2 

 to the west of Abbott's Hill, it has not been found possible to ascertain 

 the position of No. 2 in the sequence of eruption. Thin sections show— 



Phenocrysts. — Occasional crystals of olivine, and more rarely of augite. 

 The former is idiomorphic, and usually has a border of iddingsite. 



Groundmass. — The groundmass contain a great abundance of violet- 

 coloured irregular grains of augite. The feldspar is labradorite which 

 approaches a lath shape. Olivine is not common. 



Basalt No. 3. 



Basalt No. 3 is found above basalt No. 1 on Kaikorai and Abbott's 

 Hills. There are no outcrops — its position and extent have been deter- 

 mined from bonlders. The weathered rock has a distinctive steel-grey 

 colour. Under the microscope, slices show — 



Phenocrysts. — Labradorite (1-6 mm. X 0-8 mm.) often containing mag- 

 netite in the centre. Olivine allotriomorpliic, and changed along fracture- 

 lines to serpentine. Not infrequently a border of iddingsite is present. 

 There are a few idiomorphic crystals of augite of a light-brown colour. 

 A cross-section of a crystal showing prisms and both pinacoids has 

 inclusions of microlites of feldspar. 



Groundmass. — The groundmass has feldspar laths (labradorite) (0-7 mm. 

 X 0-24 mm.) set in a mass of very fine-grained feldspar. Grains of olivine 

 and augite are not common. 



The composition of the rock is as follows 



99-23 



