ON THE ROCKS OF THE HAURAKI GOLD-FIELDS. 253 



Pyrites is found in small quantity in the Tapu rock, but does 

 not occur in those from the Thames or Coromandel. 



Augite Andesite. — These rocks form the bulk of the auriferous 

 series at the Thames, Te Aroha, and at Coromandel. When 

 unaltered they are greyish-black, sometimes with a semi-vitreous 

 lustre, and a specific gravity of about 2.7. As they decompose 

 they pass through dark greenish-grey, light greenish-grey, and pale- 

 grey to white, more or less coloured with limonite. None of them 

 are markedly porphyritic. but as they decompese they get more or 

 less spotted with opaque white leucoxene. The specific gravity 

 varies with the amount of iron present, but it falls as the minerals 

 get hydrated, until in my most decomposed specimen (but one 

 sufficiently hard to grind) it is only 2.153. 



Only two of my specimens are sufficiently unaltered to have 

 any augite remaining. Both are from the road up the Waioron- 

 gomai Creek, Te Aroha, to the New Era Battery. One of them 

 is from just above the Te Aroha Batteiy, and is, no doubt, the 

 No. 2 of Mr. Cox's Report [he. cit. p. 16) ; the other was collected 

 about a quarter of a mile higher up the road, and is probably 

 No. 4 of Mr. Cox's Report. They are much alike, and may be 

 parts of the same rock. 



The ground-mass is abundant, of a colourless glass with small 

 specks of magnetite and numerous felspar microlites, which shew 

 no fluxion structure. The one from the lower level has chloritic 

 infiltrations in the ground-mass. 



The felspars are plagioclase. generally clear, but some much 

 clouded in the centre. They are zoned with small liquid inclusions 

 and generally contain grains of magnetite. Crystals cut nearly 

 at right angles to the brachypinacoid gave the following extinction 

 angles with I'eferenceto the twinning plane : — (a) 7° and 8°=:=15''; 

 (b) 20° and 2r=4r; (c) 32° and 33°=65°. The last is probably 

 bytownite or anorthite. Binary twins also occur. The microlites 

 in the base give extinction angles between the two lamella3 to 65°, 

 consequently they are anorthite. 



The augite is pale green and pleochroic, often showing poly- 

 synthetic twinning. Sometimes it is in square prisms with the 

 prismatic faces badly developed, at others in octagonal prisms with 

 the prismatic faces and pinacoids equally developed. Prismatic 

 cleavage is generally apparent. There are very few inclusions 

 except small needles of apatite. 



All my other specimens are more or less decomposed. They 

 come from Te Aroha (No. 5 of Mr. Cox's Report), from the 

 Karaka and Waiotahi Creeks at the Thames, and from Omaru 

 Bay, Coromandel. 



