Vol. 55.] RHrOLITES OF THE HAURIKI GOLDFIELDS. 455 



Hg [6]. Omahu. — This is a pale bluiah-grey lithoidal rock, with 

 indications of a spherulitic structure. 



The section shows under the microscope porphyritic crystals of 

 sanidine, andesine, and labradorite, all more or less corroded, so 

 that none of them present sharp angles and edges. The rock in 

 which these crystals occur consists of two generations of spherulites 

 (as in H^, already described on p. 452), the earlier micro felsite- 

 spherulites being of the large brown type, while those of the 

 second generation are nearly or quite colourless, and of the radiating 

 crystalline-fibrous type common in acid lavas. Both kinds of 

 spherulites are impregnated with globulites, and in the smaller ones, 

 when seen in very thin section, the globulites appear to form 

 margarites disposed obscurely in a radial manner. The large brown 

 spherulites rarely show any perfectly isotropic matter, the original 

 glass having been almost wholly devitrified by globulites, trichites, 

 and apparently fibres which may have resulted from the coalescence 

 of irregular lines or streams of globulites and longulites. Where a 

 porphyritic crystal is enveloped in the brown material these lines 

 are seen to sweep round it, indicating an ordinary fluxion-structure 

 (PI. XXXIV, figs. 1 & 3). The slide is in fact made up of sections 

 of the arms, or, as Rosenbusch terms them, the * pseudopodium- 

 like processes,' of these brown microfelsite-spherulites and the 

 colourless spherulites of the second generation. The porphyritic 

 crystals of felspar are more or less corroded, and were evidently 

 floating in the magma before the microfelsite-spherulites were 

 formed, since they are sometimes enveloped by the latter. Nests 

 and patches of tridymite occur in many parts of the section, and a 

 small amount of quartz is present either in isolated grains or 

 forming little mosaics. 



H^ [109]. Omahu. — This is a yellowish-white, lithoidal rock, 

 containing small lithophysse ranging up to | inch in diameter. 

 They show a concentric shelly structure, the interspaces between 

 the shells being hollow, whil^ the inner surfaces have a rather deep 

 brown colour, and often show a roughened or punctate appearance 

 under a pocket-lens. 



The section of this rock passes through portion of a lithophyse 

 as well as through large brown microfelsite-spherulites and small 

 spherulites of the ordinary type. Cumulites occur in irregular patches 

 in the areas occupied by the small spherulites, as also in other parts 

 of the section. On one margin of the preparation there is a brown 

 spherulitic band, the spherulites being of the micro felsi tic type. 

 They show very irregular dark brushes between crossed nicols, and 

 the band has to some extent been broken up, small spherulites of 

 the ordinary type filling the fissures (PI. XXXII, figs. 5 & 6). 

 These ordinary spherulites, which are of very small dimensions, 

 together with the large brown microfelsite-spherulites and the 

 lithophysse, constitute the entire rock. 



A good example of one of the large microfelsite-spherulites occurs 

 in this section. The centre consists of a spherulitic aggregate 



