262 Tra)isactions. 



petrographical descriptions by Professor W. J. SoUas, with additional 

 matter and micro-photographs by Mr. Alexander McKay, nine rocks from 

 Waihi mines are described. Of these, one is classed as a doubtful andesite, 

 two as'pyroxene-rhyolites or andesites, five as rhyolites (mostly j)yroxene- 

 rhyolites), and one is called " a much-decomposed altered quartz-feldspar- 

 pyroxene rock." This last rock may possibly belong to the andesites 

 overlying the auriferous series. 



Sollas also describes various andesites and rhyolites from the neigh- 

 bourhood of Waihi. Reference will be made to some of his descriptions 

 on a later page. 



In 1908 Dr. J. M. Bell and Mr. Colin Fraser, in an article on the Waihi 

 Mine (12), refer to the mine-rocks as altered dacites, containing stringers 

 of calcite, quartz (both chalcedonic and highly crystalline), orthoclase 

 (variety valencianite) in minor amount, and pyrite. They remark, " The 

 vein-bearing rocks have been described as rhyolites, but careful chemical 

 and 'petrographical investigation have led the writers of the present paper 

 to classify them as dacites " (12, p. 388). 



In the same paper reference is made to the younger andesitic and dacitic 

 lavas and tuffs that overlie the vein-bearing dacites. Three types of 

 rhyolite — namely, {a) spherulitic rhyolite, (6) pumiceous brecciated flow 

 rhyolite (" wilsonite "), and (c) grey lithoidal rhyolite — are recognized. 



Dr. J. Malcolm Maclaren, who is well acquainted with the Hauraki 

 goldfields, has also questioned Sollas's determination of the country in the 

 Waihi Mine as a " hornblende-pyroxene-rhyolite." After quoting Sollas's 

 description (11, vol. 2, pp. 67, 68), Maclaren remarks, " In view of the 

 occurrence of orthoclase (valencianite) in the lodes of Waihi, and of the 

 exceedingly altered state of the country, it is conceivable that the ortho- 

 clase found in the above rock may be valencianite due to secondary action ; 

 indeed, considerable indication of such a growth is outlined in the fore- 

 going petrological description [Sollas's]. It is therefore probable that the 

 highly decomposed rocks of the W^aihi area do not represent original 

 rhyolites, but a local succession of andesites, dacites, and even more acid 

 rocks that have been so thoroughly altered by solfataric solutions that 

 many of their original characters have disappeared " (13, p. 315). 



In 1909 Mr. A. M. Finlayson, in an article entitled " Problems in the 

 Geology of the Hauraki Goldfields, New Zealand " (14), remarks on tlie 

 occurrence of valencianite (adularia) as a secondary product in the com- 

 pletely altered rocks of Waihi. He isolated and analyzed the mineral, 

 with the following results : — 



SiO^ .. .. .. .. .. 65-85 



AI2O3 .. .. .. .. .. 1848 



K2O .. .. .. .. .. 11-25 



Na,0 .. .. .. .. .. 4-11 



99-69 



Specific gravity . . . . . . . . 2-61 



Finlayson says, " In view of the fact that this orthoclase is, in the 

 specimens examined, of secondary origin, while the primary feldspars are 

 soda-lime varieties, the original rocks appear to have been in the main 

 andesites and dacites The presence of soda in the Waihi 



