342 Transactions. — Geology. 



Art. XLI. — Notes on a Hypersthene Andesite from Waihi 



Mine, Waihi. 



By James Park, F.G.S. 



[Read before the Auckland Institute, 15th October, 1900.] 



This rock was cut through in the south-east crosscut from 

 No. 1 shaft, at the 300 ft. level of the mine, on the hanging- 

 wall of the Martha lode, near the Grand Junction boundary- 

 line. It approaches close to the surface of the old valley 

 which existed to the east of the Martha Hill before the 

 eruption of the rhyolites which cover the plains and wrap 

 round the Eosemont, Amaranth, and Black Hills, and sur- 

 round the Martha Hill on all sides, excepting the narrow 

 neck of andesite on the north side, which connects it with 

 the great andesitic area of the Hauraki Goldfields. It occurs 

 as an undecomposed bar or core passing insensibly into the 

 soft decomposed andesite or propylite enclosing the Martha 

 and Empire lodes. 



The colour of the rock is greyish-black ; when wet, 

 dense black. In hand specimens phenocrysts of feldspar 

 and pyroxene are prominent. Fracture splintery, subcon- 

 choidal. Under the microscope the essential minerals are 

 seen to be plagioclase and pyroxene, with numerous square 

 and rod-shaped microliths set in a glassy base. The base: 

 is not abundant, is mostly clear, and dusted evenly with a 

 black-brown or black dust, apparently magnetite. 



The feldspars are fairly fresh and glassy, and occur both 

 as short tabular phenocrysts, up to 5 mm. long, and as narrow 

 laths. The former are well developed, and show very marked 

 polysynthetic twinning, and sometimes zonal structure. The 

 smaller feldspars are often irregular in form and not much 

 twinned. The effects of corrosion by the cooling magma are 

 clearly seen in the rounded angles and indentations of some 

 of the feldspar plates. Binary twin crystals occur in each 

 slide. Their straight extinction indicates orthoclase. 



The indentations and cracks of the larger feldspar plates 

 are filled with calcite and fine magnetite dust, which often 

 shows oxidation to haematite. In some of the phenocrysts- 

 enclosures of colourless square-shaped microlites are common, 

 in many cases zonally arranged. 



Hypersthene is abundant and generally much decom- 

 posed, showing alteration into serpentinous products, quartz, 

 and magnetite. In some cases the crystal is almost re- 

 placed by magnetite ; in others — perhaps the more common 



