Morgan. — The. Igneous Rocks of Ihe Waihi Goldfeld. 263 



valencianite is doubtless due to its derivation from soda-feldspars by the 

 action of the potash-bearing vein-solutions " (14, pp. 634-35). 



Finlayson also made a series of instructive analyses of specimens, all 

 except one of which were from a crosscut to the Empire vein at the 850 ft. 

 level of the Waihi Mine. These analyses show clearly the transition from 

 " chloritized hornblende-dacite," with 58-39 per cent, of silica, to " altered 

 dacite," with 61-78, 69-35, and 76-61 per cent, of silica, and finally to 

 " replacement ore," with 85-65 per cent, of silica. 



In his latest work Professor Park refers to the rocks of the Waihi 

 Goldfield in much the same terms as in 1897. The auriferous rocks are 

 called " altered andesites and dacites " (15, p. 349). 



In a recent paper, entitled "The Waihi Goldfield" (16), Dr. J. M. Bell 

 mentions the Waihi rocks by the same names as in the previous article 

 by himself and Mr. C. Eraser (12). 



Special Petrography. 



From February, 1903, to May, 1905, the writer made and microscopic- 

 ally examined a large number of sections from specimens of rocks occur- 

 ring in or near the Waihi district. These will be described and discussed 

 under the foUowmg headings : — 



(1.) Rocks of the auriferous series. 



(2.) Stratified tuff of the Grand Junction shaft. 



(3.) Andesites and tuffs of the " Second Period." 



(4.) Rhyolites. 



(5.) Andesitic rocks of doubtful age. 



(1.) Rods of the Auriferous Series, 

 (a.) Quartz-andesites and Dacites. 



The specimens from which sections were made are for the most part 

 from Nos. 5 and 6 levels* of the Waihi Mme. These levels correspond 

 to depths below the surface of 445 ft. and 555 ft. respectively as measured 

 from the collar of No. 1 shaft. Three or four are from higher levels of the 

 Waihi Mine, and a few come from the Grand Jmiction and Waihi Extended 

 Mines at depths of approximately 480 ft. to 500 ft. below the surface. 



Macroscopically the specimens are generally dark -grey or greenish close- 

 grained rocks, showing porphyritic feldspars of moderate size and a few 

 phenocrysts of glassy quartz. Numerous dark altered crystals are refer- 

 able to pyroxenic or amphibolic minerals. A marked effervescence with 

 cold dilute hydrochloric acid in practically all specimens indicates the 

 presence of calcite. 



Under the microscope most sections exhibit many crystals of moderately 

 or well twinned plagioclase which by the extinction angles and relative 

 indices of refraction as compared with other minerals and with Canada 

 balsam are shown to be mostly acid labradorite. Andesine is present in 

 many cases. The twinning is nearly always on the albite law : pericline 

 twinning is somewhat rarely seen. Zonary banding, pointing to a difference 

 in the composition of individual crystals, is quite common. Nearly univers- 

 ally, if not always so, the more acid feldspar (usually near andesine) is on 

 the outside. 



* These are the numbers given in published plans and reports ; at the mine itself 

 the practice is, or was, to call these levels Nos. 6 and 7. 



