268 Transactions. 



(e.) Alteration of Vein-hearing Dacites. 



From the preceding pages it will be gathered that near the quartz lodes 

 the dacites are partly silicified, especially in the groundmass. The original 

 lime-soda feldspars have been almost entirely replaced by valencianite, 

 quartz, probable sericite, and other minerals. Chloritization of the ferro- 

 magnesian constituents is prominent. In the sulphide-ore zone more or 

 less pyrite is present. 



At some distance from the veins chloritization of the ferro-magnesian 

 minerals is perhaps the most noticeable feature. Many, but not all, of 

 the lime-soda feldspars appear to be replaced by valencianite. Calcite as 

 a secondary mineral is more abundant than in the wall-rocks. Other 

 secondary minerals are quartz, a little pyrite, and perhaps some muscovite 

 and sericite. 



By the changes outlined in the last paragraph what may be called the 

 propylitic facies is brought about. The name " propylite " for such rocks, 

 as used by Park and others, is a most convenient term that midoubtedly 

 fills a M'ant, and it is therefore to be regretted that the usage is open to the 

 objection that the name was originally intended to indicate a distinct rock 

 species. 



Further discussion of the nature of the orthoclastic feldspar of the 

 altered dacites seems desirable. Some is clearly secondary, and is there- 

 fore valencianite. Finlayson has shown that this valencianite is a soda- 

 orthoclase or anorthoclase (14, p. 634). Much more abundant, however, 

 are altered feldspar crystals, now consisting partly of untwinned feldspar 

 of low refractive index, partly of quartz, calcite, and other minerals. 

 Reasons for believing that the feldspar in these crystals is not the remains 

 of the original mineral, but is an alkali (potash-soda) feldspar of secondary 

 character, are the following : — 



(1.) In many sections evident transitions from lime-soda feldspars to 

 purely alkali feldspars may be seen. 



(2.) The nearer the lodes, the less the number of recognizable lime-soda 

 feldspars, and the greater the number of alkali feldspars. 



(3.) Chemical analyses show comparatively low percentages of silica 

 and potash in the altered dacites when 30 ft. or more from any large lode. 



(4.) The alkali feldspars usually contain as a decomposition-product 

 calcite, and therefore the presence of lime in the original feldspar is indi- 

 cated. 



Of the highly altered feldspars, it cannot be asserted that all were 

 originally lime-soda feldspars, but it seems certain that at least the majority 

 were such. There may, however, be reason for thinking that the fresh 

 but rarely seen individuals of apparently pure potash feldspar with binary 

 twinning are primary. It would be difficult to prove the contrary. 



If the possibility that some of the alkali feldspar is original be admitted, 

 then it may be supposed either that orthoclase was a constituent of the 

 original andesitic or dacitic magma, or that during or immediately preceding 

 eruption there was some mixing of an andesitic magma with a rhyolitic 

 differentiate. The presence of corroded quartz seems to support the latter 

 hypothesis, which is decidedly an attractive one, however slender a founda- 

 tion it may rest upon. 



(/.) Significance of Propylitic Facies. 

 It may be asked whether the propylitic facies, as developed at Waihi, 

 does not necessarily involve the formation of secondary alkali feldspar 



