Morgan. — The lyneous Rocks of the Waihi fJohlfield. 267 



cannot be ascertained. The groundmass is more or less a quartz mosaic. 

 Other minerals present in it are magnetite, pyrite, and calcite. 



(d.) Discussion of Nomenclature. 



With the notable exceptions of Sollas, and to some extent of Lindgren, 

 the various writers who have been quoted on previous pages substantially 

 agree in describing the rocks of the auriferous series as andesites, quartz- 

 andesites, or dacites. Lindgren hesitates between dacite and quartz- 

 trachyte as a name for these rocks ; but this geologist examined wall-rock 

 from the Waihi Mine only. It would also seem to be the case that all, or 

 nearly all, Sollas's determinations were made from samples of highly altered 

 rock adjoining large veins. Had Sollas been given an opportunity of 

 examining a series of less-altered specimens taken some distance from the 

 larger ore-bodies it is probable that he would have reached conclusions 

 more in agreement with those of New Zealand workers.* As matters stand, 

 however, the presence of much apparent orthoclase has given rise to a diffi- 

 culty in naming the auriferous rocks. 



From the petrographical descriptions given on the preceding pages it 

 will be gathered that there is much evidence for the view that most, if 

 not all, of the supposed original orthoclase is of a secondary character, 

 replacing original lime-soda feldspar, as has already been suggested by 

 Maclaren and Finlayson. Thus the determination of the auriferous rocks 

 as quartz-andesites or dacites (these terms being here used as all but identical 

 in meaning) may be regarded as proved, at least as regards the Nos. 3, 4, 5, 

 and 6 levels (279 ft., 353 ft., 445 ft., and 555 ft.) of the Waihi Mine, the 

 500 ft. level of the Waihi Extended Mine, and the 494 ft. level of the Grand 

 Junction Mine. There is no reason for supposing that the rocks of the 

 Waihi-Union and Waihi-Silverton Mines are of different character. Samples 

 of the less -altered country from all the Waihi mines show a great 

 similarity, so that they are possibly all of one original type — probably a 

 hypersthene-dacite with some augite and perhaps a little hornblende. 



Chemical analyses of the less-altered mine-rocks entirely support their 

 determination as dacites of a somewhat basic type. The analyses known 

 to the writer show a silica-content ranging from 55 to 61 per cent., with 

 other constituents in proportions normal to an ordinary andesite or dacite 

 that has been somewhat affected by solutions containing carbon-dioxide. 

 The following determinations of silica and water made on rocks from the 

 present lowest level of the Waihi Mine, communicated to the writer by 

 Mr. A. H. V. Morgan, M.A., Director of the Waihi School of Mines, show 

 that the rocks at that level are, if anything, less acidic than in the upper 

 levels : — 



Moisture lost o-^-v 



at 100° C. ^^^2- 



I .. .. .. .. 2-30 55-13 



II .. .. .. .. 1-80 56-23 



I, cormtry in crosscut at 1,000 ft. level towards Roj^al reef, about 50 ft. 

 from No. 5 shaft ; II, country in crosscut at 1,000 ft. level from No. 5 shaft, 

 about 50 ft. south of No. 4 shaft. 



* Sollas, however, regarded some of the rocks he examined as possibly andesites, 

 and his descriptions of specimens 3/2719 and 4/2721 (11, vol. 1, pp. 128-50) may be 

 cited as evidence that he was inclined to call rocks similar to those from the Waihi mines 

 altered dacites or andesites. 



