488 



Tr ansae tions . — Geology. 



they appear as colourless rods or narrow plates, generally 

 twinned. The extinction angles, varying from 20° to 35°, 

 indicate that the feldspars are chiefly labradorite, with per- 

 haps a little anorthite. 



The chief accessory minerals are magnetite (probably 

 titaniferous), occurring probably as a decomposition product 

 of the augites, and carbonate of lime in small thread-like 

 veins. A httle apatite is present in the augites, and gas-pores 

 and negative crystals occur in the feldspars. 



The various constituents follow the normal order of con- 

 solidation, and the almost holocrystalline nature of the rock 

 points to its having consolidated slowly and under pressure, 

 and to its probably being a dyke rock, though perhaps at its 

 original surface (now denuded away) it may gradually have 

 merged into a basalt. 



Appended (Plate XL VI.) are two diagrammatic water- 

 colour drawings — No. 1, under ordinary light ; No. 2, under 

 crossed nicols. a = augites ; / = feldspars. 



Art. XLIV. — Notes on a Hypersthene Andesite from White 



Island. 



[Read before the Auckland Institute, 15th August, 1898.] 



By W. A. MacLeod, B.Sc. 



Plate XL VII. 



This is a rock of a dark-grey colour, irregular fracture, and 

 specific gravity 2-65. In hand specimens the feldspars are 

 conspicuous, reflecting the light from their smooth faces, 

 whilst the hypersthenes only appear as dark oblong patches. 

 The following is a chemical analysis of this rock : — 



SiOo ... ... ... ... 64-49 



AlA 



Fe.A 



Feb 



CaO 



MgO 



K.,0 



Na.,0 



Ignition loss 



The SiOa in the above is some 4 to 7 per cent. 



