MOROAN. — The I;/)icou.<< Forks of flie WaiJii Clohlfrhl. 271 



specimens from the 500 ft.* levels of the Waihi Extended and Grand Junc- 

 tion Mines they are converted almost wholly into chlorite, and elsewhere are 

 not uncommonly partly chloritized. 



In a few slides a little hornblende with resorption border is present. 

 Lozenge-shaped bodies filled with dark dust such as have been observed in 

 the older lavas are not rare and in the absence of further study may be 

 presumed to represent hornblende that has suffered almost complete resorp- 

 tion. 



The other primary minerals present in the hypersthene-augite-andesites 

 are magnetite in small grains, apatite in microscopic needles penetrating 

 phenocrysts of feldspar and pyroxene, and possibly one or two of the rarer 

 rock-forming minerals, such as zircon, in tiny grains. 



In the Grand Junction and Waihi Extended Mines alteration of the 

 Second Period rocks overlying the auriferous series is in places very marked. 

 Besides the chloritization of the ferro-magnesian minerals, the feldspars 

 may be much decomposed, with the production of calcite, quartz, and 

 " kaolinitic " matter (probably mainly sericite). Though these effects are 

 seemingly of a propylitic nature, they may be partly due to meteoric 

 solutions. In specimens from shafts and boreholes on the western side of 

 the Martha Hill much alteration is also apparent. In this area serpentine 

 is a prominent secondary mineral in the andesites. 



Since the decomposed rocks mentioned in the last paragraph are not 

 known to contain any auriferous-quartz veins, it may be supposed that 

 their alteration was not associated with the formation of ore-bodies. The 

 question deserves investigation, however, for the alteration may possibly 

 have taken place during a period when the lodes in the underlying auri- 

 ferous series were being enriched, or at a time when values in these lodes 

 were being concentrated in ore-shoots. 



In the hills towards the coast-line north-east of Waihi are several varieties 

 of andesitic rocks that differ from the ordinary hypersthene-augite-andesite. 

 These, since they occur somewhat outside the area intended to be described 

 in this paper, will not be further mentioned. 



(6.) Tuffs. 



On the top of a. hill somewhat more than a mile north-west of the 

 Martha Hill is a small outcrop of coarse tuff underlain by a thin clayey 

 layer, below which comes an ordinary andesite. A section made from 

 one of the boulders shows that it is a vesicular hornblende-andesite, with 

 some augite. The vesicles are lined by a yellow undetermined substance. 

 Another boulder proved to be an andesite with beautifully twinned augite 

 and a slightly pleochroic rhombic pyroxene (bronzite). It is quite possible 

 that the tuii under notice belongs to a later period than is here assumed, and 

 ought to be associated with the hornblende-andesites described on page 273. 



The andesitic tufTs that appear some miles north-east of Waihi and 

 along the coast-line have not been examined in detail by the writer. 



(4.) Rhyolites. 



As already mentioned, three types of rhyolite occur near Waihi — one 

 spherulitic, the second (wilsonite) with a peculiar flow structure, and the 



* In the Grand Junction Mine the exact depth is 494 ft. below the collar of the 

 A or No. 1 shaft. 



