Vol. 56.] EKUPTTVE KOCKS FROM NEW ZEALAND. 493- 



25. Additional Notes on some Ercttive Eocks from New Zealand. 

 By Frank Ecttley, Esq., F.G.S. (Eead April 4th, 1900.) 



[Plate XXVII.] 



The rocks described in this paper were, with two or three excep- 

 tions, collected by Mr. James Park, F.Gr.S., and reached me just 

 before the completion of the paper on the rhyolites of the Hauraki 

 Goldfields, read last year before this Society. 1 A few of these 

 specimens came from the area dealt with in that paper, but a con- 

 siderable number of them are from other localities in the North 

 Island, including several from Eotorua, which are interesting as 

 affording recent examples of solfataric action. 



For the estimations of the total silica in these rocks I have to 

 thank my friend Mr. Philip Holland, F.I.C., F.C.S., by whom they 

 were made. Lantern-slides of some of the more typical rock- 

 sections have been very kindly prepared for me by Mr. Frederick 

 Chapman, A.L.S., F.E.M.S., of the Eoyal College of Science, South 

 Kensington. 



The present paper may, to some extent, be regarded as a con- 

 tinuation of that on the Hauraki rhyolites, but although certain 

 specimens, about to be described, bear a tolerably close resemblance 

 to some of those dealt with in the last paper, many of them, when 

 examined microscopically, are found to present points of interest 

 which render them worthy of special notice. 



These rocks vary so much in their structural details that no 

 attempt will be made to adopt any systematic order in their 

 description, except that the specimens from Eotorua will be treated 

 consecutively at the end of the paper. 



H 21 . Waihi. — A pale-grey, lithoidal rhyolite, with little 

 dark-green crystals suggestive of pyroxene, and small colourless 

 crystals of felspar. 



Under the microscope the rock is seen to be a rhyolite with an 

 irregularly-undulating fluxion-banding, the lithoidal character being 

 due to the development partly of globulites, partly of spherulitic 

 growths. The latter are best seen in ordinary transmitted light, 

 when magnified about 150 diameters. Between crossed nicols the 

 spherulitic structure is very obscure and, owing to the bright 

 illumination of only a few fibres or rods in each spherulite, gives 

 rise to the appearance of a rather sparse dissemination of microlites 

 through a partly isotropic groundmass. Small porphyritic crystals 

 and fragments of augite are present in the rock. The prismatic 

 angle, measured in a section normal to the vertical axis of one 

 of these crystals, was 87°. The augites are more or less corroded. 



1 'Notes on the Rhyolites of the Hauraki Goldfields' by J. Park & 

 F. Kutley, with Chemical Analyses'by P. Holland, Quart. Journ. Greol. Soc. 

 vol. lv (1899) p. 449. 



Q.J.G.S. No. 223. 2 l 



