CoTTOi^. ^Geology of Signal Hill, Dunedin. 



123 



Phenocrysts. — The phenocrysts are augite, olivine, feldspar, and magne- 

 tite. Of these, olivine and augite are by far the most abundant. The 

 olivine crystals are large, and are partly altered to serpentine. They are 

 almost colourless. The augite crystals are grey in colour and up to 

 J mm. in breadth. Thev have no pleochroism, and the extinction-angle 

 is 41°. 



Feldspar phenocrysts are rare. They are of basic labradorite, chiefly 

 untwinned, and are about 1 ram. in length. 



Magnetite grains occur up to 0-1 mm. in diameter. 



Groundmass. — The groundmass is holocrystalline, and is coarser-grained 

 than in the other basalts .described. It consists chiefly of allotrimorphous 

 grey augite, with labradorite laths averaging 0-3 mm. in length. The 

 extinction-angle on either side of the albite twin line in sections perpen- 

 dicular to 010 is 32^°. There are also abundant grains of magnetite. No 

 glass is present. 



This rock, owing to the scarcity of feldspar and abundance of augite 

 and olivine, approaches a magma basalt or augitite. 



Chemical Composition. — The chemical composition proves that the 



rock is derived from an extremely basic magma. For comparison the 



analysis of a basic basalt from Punta Delgrada, San Miguel, Azores,* is 



quoted : — 



Dyke Basalt, Basalt, Punta Del- 

 No. 3, grada, S. Miguel, 

 Signal Hill. 



SiO, 



AUO3 



Fe,03 



FeO 



MgO 



CaO 



Na^O 



KoO 



H.,0 



Tib., 



PoO, 



CI 



MnO .. .. .. .. .. 0-36 



0-11 



Basalt No. 4. 



This basalt occurs in fragments in a scoria- bed on the Signal Hill 

 Road, lying above the Logan's Point phonolite and below the main basalt- 

 flow. 



Macroscopic Characters. — The structure is porphyritic. the matrix being 

 grey and fine-grained, with numerous white phenocrysts of feldspar and 

 occasional black specks of augite. 



Microscopic Characters. — In sections the porphyritic structure is strongly 

 marked, augite and feldspar occurring in two generations. No flow structure 

 is discernible. 



*€. V, John, Jb. G. R.-A., Wien, xlii, p. 291, 1896. 



