BY D. MAWSON. 459 



6. Hornblende audesite. 



Locality. — As a boss quite 50 yards in diameter intruding 

 the Miocene limestones and tuffs on the Wai Malikoliko at its 

 junction with Dry Creek. 



Macroscopic Character s. — Colour light ashy-grey. 

 Fracture uneven. Texture line-grained with base. Co7tstituents 

 indefinite, slightly yellowish-stained felspars are most numerous, 

 but less conspicuous than tiny black prisms of liornhlende. 



Microscopic Characters (Plate xix., fig.2) — Texture : 

 Crystallinity, pauidiomorphic hypohyaline. Grain-size variable, 

 averaging 0*5 mm. 



Minerals present (in approximate proportions by area) : — 



Glass— greyish and dusty 58% 



Felspar— acid labradorites (AbgAn.^-AbjAn^) ... 30% 



Hornblende — light yellowish-green 7% 



Pyroxene— almost colourless ... ... ... ... 2 '5% 



Magnetite — line grains 



Apatite— a minute quantity only i 



2-5% 



100-0 



Base. — The vitreous appearance of the glass is relieved by the 

 presence of much fine dusty matter, resulting in a general 

 yellowish-grey colour; in addition, abundant microlites and 

 cryptoscopic fragments of the constituent minerals help to 

 increase the turbidity. 



Felspar appears in the usual idiomorphic rectangular prisms, 

 averaging 1 mm. in length. They are much zoned, dusty areas 

 of acid labradorite (Ab^ An^) alternating with clearer bands of 

 andesine (Abg An^). Some of the more dusty zones closely 

 resemble the glassy groundmass without, and it appears as if, 

 during the growth of the crystal, envelopes of the base material 

 had occasionally been sealed up within further additions of the 

 crystal substance. 



Hornblende appears in perfectly idiomorphic small even-sized 

 crystals, O'l to 0-3 mm. diam. Multiple twinning on a plane 

 parallel with the a face was observed in several cases. It is of a 



