BY W. G. WOOLNOUGH. 515 



A ugite Andesite (Namulowai). Plate xxxv., figs. 4-5. 



Macroscopic characters. — In hand specimen the rock is dark 

 blue-grey in colour, and very compact in texture. It consists of 

 a well-marked cryptocrystalline base, with very numerous, beauti- 

 fully fresh, small, felspar phenocrysts,in which the albite striations 

 can be readily seen with a lens. Much less abundant than the 

 felspar is augite, in rather dark greenish-yellowish crystals, or 

 nests of crystals. There is no distinct evidence of flow. 



The outcrop described in Part i. of this paper is a remarkable 

 thimble-shaped hill. It is very roughly columnar, so that the rock 

 tends to break up into angular fragments from 4 or 5 inches in 

 diameter upwards. Specific gravity, 2-64. 



Microscopic characters. — The base consists of a light yellowish- 

 brown glass, through which are scattered very numerous crystal- 

 lites and microlites, giving it a characteristic hyalopilitic texture. 

 The crystallites have the form of very minute straight or curved 

 rods and fibres, and are irregularl}^ interlaced without any obvious 

 flow-structure. 



Through the glass are scattered — but not abundantly— very 

 minute microlites of felspar. These are square to lath-shaped 

 sections, whose extinction is sensibly straight. Ko trace of 

 twinning is to be seen. The characters observed agree, so far as 

 they go, with oligoclase. 



Still more scarce than the felspar microlites are those of augite. 

 They take the form of rather slender prisms, with a faint yellowish 

 colour, high refractive index, and large extinction angles. 



One of the most remarkable features of the rock is the abun- 

 dance of felspar phenocrysts. These vary in size uj) to about 

 1-75 mm. long. A few show perfectly sharply defined outlines, 

 but most have more or less strongly marked resorption rims. 

 All stages can be traced, from perfect crystals to mere haz}^ 

 patches, where the base is lighter in colour than usual, marking 

 places where felspar crystals have been almost completely redis- 

 solved. 



