ON THE ROCKS OF THE HAURAKI GOLD-FIELDS. ZD^ 



Apatite occurs in small quantity in the chlorite pseudomorphs. 



Calcite gets more and more abundant in the felspars as decom- 

 position proceeds, but in the last stages of decomposition it is 

 removed, and the place of the felspars left vacant. The rock then 

 easily falls to pieces. Occasionally calcite can be detected in the 

 chlorites. 



Gypsum has been detected by Mr. Skey in some of these rocks, 

 but I have failed to recognise it, unless possibly it may form the 

 colourless aggregate in some of the chlorites. 



Quartz (secondary), generally occurs in the ground-mass, and 

 increases pari passic with the calcite. In one specimen from Te 

 Aroha a fine granular brownish aggregate, with brilliant polari- 

 zation colours, is found in the felspars and augites, which is 

 probably chalcedony ; it is accompanied by a colourless isotropic 

 mineral, which is no doubt opal. 



Titaniferous magnetite is present in the less decomjDosed rocks, 

 but usually it is more or less altered into leucoxene, and ultimately 

 entirely into that mineral, or occasionally into hsematite or 

 limonite. In one case however the iron oxides remain unchanged, 

 although the felspars and augites are entirely decomposed and 

 filled with chlorite, chalcedony, and calcite. Secondary magnetite 

 sometimes occurs in connection with chlorite patches in the ground- 

 mass, or as narrow black borders to the decomposed augites. 



Pyrites occurs both in grains and in crystals, but varies very 

 much in quantity ; being abundant in some rocks, in others quite 

 absent. This is not due to decomposition for often it is abundant 

 in the most decomposed rocks, while in the little decomposed rocks 

 from Waioi-ongomai it is altogether absent. Neither is it due to 

 locality ; for often rocks with plenty of pyrites, and others with 

 none, may be found in close proximity at the Thames, at Te 

 Aroha, and at Coromandel. In one specimen from Waiotahi 

 Creek pyrites occurs only as minute specks in the magnetite. In 

 other cases it is not uncommon to find pyrites closely associated 

 with leucoxene, which is then generally of a yellowish colour. 

 This looks as if the pyrites had been formed from the titaniferous 

 magnetite. 



ExsTATiTE Andesites. — These rocks when unaltered are greyish- 

 black in colour, but as alteration progresses they change to a dark 

 greenish-grey. In the former state they are the dolerites and in. 

 the latter the melaphyres of my report of 1868-9. On further 

 decomposition they pass into lighter colours, but they cannot then 

 be distinguished from the decomposed augite andesites. The 

 specific gravity of the least altered rock varies from 2.72 to 

 2.80 ; in the more altered examples it sinks as low as 2.636. 

 They are found at the Thames in Karaka, Collarbone, Waiotahi, 

 Moanataiari and Tararu Creeks, as well as in deep levels of the 

 Prince Imperial Mine : and they also occur at the Waiorongomai 

 River, Te Aroha. 



