56 KOTO : NOTES ox THE GEOLOGY 



the magma, so that there remains nothing but the accumulation 

 of magnetite-dust in the place of the hornblende. 



A brown pleochroic hypersthene occurs in few quantity, and 

 small in size and less idiomorphic when compared with the 

 amphibole. Its base shows no axial poles, but symmetrical 

 hyperbolas ; it forms penetrating twins upon a domal face, and 

 often moulded upon plagioclase. The plagioclase is of tabular 

 or long-rectangular shape ; extinguishes symmetrically in the 

 direction at about 30° against the trace of the albite-twins, and 

 the trace of pericline lamellœ makes -5° to -10° on 010 with P/M, 

 indicating the presence of labradorite. The albite-lamallse are 

 clear and definite, but the width varies much from one lamella 

 to another, and even in the same the width varies from one 

 point to another, — these are also said to characterise labra- 

 dorite. Zonary banding is pronounced, the interior abounds in 

 glass-enclosures, with the clear shell of different optical orien- 

 tations. AVe meet often with the lu'oken crystals, from which it 

 may be inferred that the rock is a lava-flow. The ground-mass 

 consists of a plexus of augite- needles in a colourless base, inter- 

 mixed with a somewhat lai-ger plagioclase of a tabular, or long- 

 rectangular form, after the manner of a micro-phenocryst. Twin- 

 ned slender sections show symmetrically the opposite extinction 

 at an angle of 20°, indicating that the felspar in the ground- 

 mass is andesine in lieu of the larger, phenocrystic labradorite. 

 3Iagnetite abounds in the glassy base. Tridymite fills free spaces 

 in imbricated scales. 



