176 



Transactions. 



(b.) Junction Basalt. 



The occurrence of this rock over an area around the Junction School 

 has already been noted. 



In hand-specimen the Junction basalt is similar to the Mount Holmes 

 rock, but breaks with a much less regular fracture than the latter. 



Under the microscope the chief difference is seen to be in the structure. 

 The groundmass is dense and microcrystalline ; it consists of predominant 

 magnetite in small squares, plentiful colourless augite granules, and inter- 

 stitial microlitic grains of feldspar. 



Large laths of feldspar showing albite twinning are fairly plentiful, 

 and, with olivine and augite, comprise the phenocrysts. The variety of 

 feldspar is chiefly labradorite. Olivine and augite are in large very 

 plentiful crystals. Celyphitic arrangement of the augite about the olivine 

 is not infrequent. The augite is a pink variety, and the olivine often 

 shows alteration to serpentine and to carbonates. 



A few large crystals of ilmenite and magnetite are present. 



A curious feature of the rock is the occurrence of occasional large 

 crystals of nepheline that have suffered considerable resorption ; there 

 is a wide fringing zone of small feldspars and a central remnant of the 

 nepheline. The nepheline has probably been caught up from contiguous 

 rocks. It is comparatively plentiful in a rock found on the hillside north- 

 west of the North-east Valley tannery — a basalt characterized by abundant 

 feldspar and sharply idiomorphic violet augite phenocrysts, and by a 

 very dense groundmass — and has there the same peculiarities. 



The order of crystallization in the Junction basalt seems to be : Pheno- 

 crysts (in order), iron-ore, olivine, augite, feldspar, and then (in order) the 

 magnetite, augite, and feldspar of the groundmass. 



Chemical Characters. 



An analysis of the Junction basalt is appended, with, for purposes 

 of comparison, analyses of two other basalts. The analysis shows high 

 percentages of silica, magnesia, and ferrous iron, which correspond well 

 with the abundance of augite in the rock. 



Si0 2 ' 



A1 2 U 3 



Fe 2 3 



FeO 



MgO 



CaO 



K 2 



Na 2 



H 2 



CI 



Ti0 2 



P 2 6 



NiO 



Ba() 



99-50 



99-86 



100-26 



A. Basalt No. I* 



B. Junction basalt. Mount Cargill. (Analysis, J. Bartrum.) 



C. Scoriaceous basaltic lava from recent eruptions at Pantellaria.f 



* Cotton, " (ieology of Signal Hill," Trans. N.Z. Inst,, vol. 41, 19C9, p. 122. 

 f H. S. Washington, " Titaniferoua Basalts of the Western Mediterranean," Quart. 

 Journ. Geol. Soc, vol. 63, Feb., 1!>07, p. 75. 



