466 'messes, j. paek and p. rutley on the [Aug. 1899, 



Another interesting point with regard to the Hauraki rhyolites 

 is the general preponderance of plagioclastic felspars 

 among the porphyritic crystals, which might almost induce 

 one, at first sight, to regard them as allied to dacites. Mr. Philip 

 Holland's analyses, however, seem to indicate that the series is rhyo- 

 litic, and it is quite possible, and in some instances almost certain, 

 that some of the more basic porphyritic crystals are derived from the 

 andesitic rocks which occui' so plentifully in the neighbouring districts. 



It is also interesting to note the silicifying effects of hydro- 

 thermal agency upon some of the rocks, and that this does not 

 merely bring about the cementing of tuft's, but seems also to affect 

 solid lavas in a very marked manner. The analysis of some of 

 these rocks would probably show an abnormally high percentage of 

 silica, as in the case of an altered rhyolite collected by W. H. Weed 

 at Iron Pot, Lower Basin, which on analysis by J. E. Whitfield 

 yielded as much as 89*2 per cent, of silica..^ Such super-silicification, 

 if met with in any of our ancient rhyolites, would go far to prove 

 the former presence of hot silica-bearing springs in this country. 



In describing the rhyolites of Omahu, considerable space has been 

 devoted to the bran ching spherulites which they contain. This 

 has been done, partly on account of the interest attaching to these 

 bodies, partly in order to give, if possible, a clear account of the nature 

 of the microfelsitic matter of which they are composed. That these 

 spherulites consist of an imperfectly-devitrified glass has been 

 indicated by Vogelsang when describing similar spherulitic struc- 

 tures in the rhyolite of Tolcsva, and from an examination of the 

 rhyolites of Omahu it has been possible not only to verify all that 

 Vogelsang has written about the Hungarian spherulites, but what 

 E-osenbusch has stated regarding the nature ofmicrofelsite.^ This , 

 it appears, is a convenient name for a rudimentary form of devitri- 

 fied glass. The devitrification consists in the development of 

 something resembling fibrous structure, but these so-called fibres, 

 when highly magnified, have the appearance of imperfectly-defined 

 strings of margarites, with an occasional tendency to assume a longu- 

 litic aspect. Among these fibres, which frequently assume an approxi- 

 mately parallel arrangement, there often lie some small irregularly- 

 shaped filmy scales, almost suggestive of tridymite. Globulites 

 and trichites may also be present. The latter, however, do not 

 appear to be essential constituents of the microfelsite, but rather 

 independent developments. 



It has been questioned by some petrologists whether the term 

 microfelsite be worthy of retention ; but, if rightly understood, 

 it appears to be of value in denoting a phase of devitrification, not 

 adequately to be expressed by any other name current in petro- 

 graphy, while its abandonment would produce a gap which could 

 be bridged only by much circumlocution. 



Within the limits of this paper it would scarcely have been 

 possible to do justice to the many observers who have treated of the 



^ ' Analyses of Eocks & Analytical Methods,' Clarke &Hillebrand, Bull. U.S. 

 Geol. Surv. 1897, No. 148, p. 132. 



=* ' Mikroskop. Physiogr.' 3rd ed. vol. ii (1896) pp. 583-594 et seqq. 



