22 Transactions. 



hot springs overflow on the surface they form thick, mush- 

 room-shaped mounds of silica. The silica is sometimes soft 

 and porous, and often dense, hard, and chalcedonic. In all 

 cases the hot springs and geysers are grouped around the 

 volcanic vents, and along fissures in lavas near the point of 

 emission. The waters range from strongly alkaline to acid ; 

 and at Eotorua alkaline and acid springs exist side by side. 

 The ascending deep-seated waters are strongly alkaline ; while 

 the source of the acid waters is the superficial deposit of 

 pumice which overlies the rhyolite. The pumice in some 

 places contains disseminated marcasite pyrites, and where 

 the alkaline waters come in contact with the pyrites they 

 are oxidized and reach the surface either neutral or acid, 

 according to the degree of oxidation. 



In the Hauraki gold-mining area, which adjoins the 

 northern end of this volcanic region, the country rocks consist 

 of a vast pile of andesitic lavas, tuffs, and breccias of younger 

 Tertiary age, resting on slaty shales and greywacke of pro- 

 bably Triassic age. The gold-bearing veins traverse both the 

 andesites and tuffs, but are only productive in the former. 

 They are fissure-veins ; but, strictly speaking, they do not 

 conform to the usually accepted definition of a true fissure- 

 vein, since they are generally confined to the igneous -rock 

 formation. Near the borders of the andesites the veins are 

 small and unimportant, and generally die out when they reach 

 the underlying basement rock. On the other hand, the larger 

 and more productive veins are grouped around the old vents, 

 and there seems to be no reason why they should not descend 

 to great depths. In opposition to this view Professor Beck ::: 

 states that it is inconceivable that mineral deposits could be 

 made from solutions at great depths. The country rock on 

 the walls of the ore-veins is propylitised to a moderately hard 

 grey rock. When two or more veins run parallel with each 

 other, as they do in all the Hauraki mining centres, the 

 country rock between the veins is often entirely altered, or 

 propylitised. 



In the Thames district the distance between the numerous 

 parallel veins which traverse the goldfield seldom exceeds 

 200 yards, and in almost every instance the veins are sepa- 

 rated from each other by a narrow belt of hard unaltered 

 andesite. These hard bands, or "bars" as the miners term 

 them, possess the same general strike and dip as the veins, 

 and in cross-section present the appearance of lenticular and 

 hourglass - shaped masses. They vary from a few feet to 

 30 yards in width. The country rock has been found to be 

 propylitised down to a depth of nearly 1000 ft. below sea- 



* Prof. Beck, " Lehre von den Erzlagerstatten," 1901, p. 139. 



