Vol. 56.] ERUPTIVE ROCKS FROM SEW ZEALAND. 49^ 



that there is a little nearly isotropic matter present among the 

 grains of quartz and felspar : the seemingly isotropic matter being 

 seen under a higher power (140 diameters) to be filled with globulites 

 and margarites, and to show very decided double refraction in places. 



These apparently isotropic patches are not really so, but are 

 diminutive fragments, evidently derived from the spherulites which 

 bound this felsitic area. In order to prove that this is the case, 

 one has only to examine the margin of this small area, when it 

 will be seen that those parts of the spherulites abutting on it are 

 gnawed away and disintegrated along the fibration of the spherules 

 in the most irregular manner. Small pieces of the spherules have 

 become detached and taken up in the felsitic matter, and the latter, 

 moreover, may be seen occupying peripheral parts of spherules which, 

 where they remain intact, are almost devoid of double refraction, 

 Nearly in the middle of this felsitic area may be seen, in ordinary 

 transmitted light, a well-defined circle, from the centre of which 

 as though traced by fine pen-strokes, radial markings are partly 

 outliued. In polarized light, this ' sketch of a spherulite ' breaks up 

 into a number of irregular patches or grains, which constitute part 

 of the surrounding felsite. (See PI. XXVII, figs. 3 & 4.) 



If we endeavour to realize the various changes through which 

 this small portion of the rock has passed before reaching its present 

 condition, we have to consider : 



(1) The rise and outpouring of a rhyolitio magma containing numerous 



already-formed crystals of felspar, quartz, etc., some entire, others 

 broken, which are undergoing a process of superficial corrosion ; 



(2) As this lava solidifies rapidly, it becomes a glass or obsidian, containing 



the porphyritic crystals and fragments of earlier generation ; 



(3) At a subsequent period the glass becomes devitrified by the development 



of globulites and other microscopic bodies, and by the formation of 

 spherulites, which originate at numerous points, incorporating in their 

 growth a certain amount of the devitrified glass. These from their 

 brown colour, their dimensions, and from the prevalence of globulites 

 in them, may (from comparison with sections of similar rocks from the 

 district) be presumed to have been microfelsite-spherulites. Doubtless, 

 when first formed they would have exhibited such double refraction as 

 spherulites of this kind are generally found to possess, and, as they 

 continued to increase in size, the growth of their radiating fibres would 

 become arrested by those of adjacent spherulites, in some cases along 

 planes tangential to the mutually-approaching spheres, thus giving rise 

 to the polygonal forms which the spherulites sometimes assume ; 



(4) Conditions supervened, such as may be accounted for in any active 

 volcanic district, which resulted in this solidified spherulitie rock being 

 again raised to a high temperature, one sufficiently high to deprive of 

 their double refraction, not merely the spherulites, but also the earlier- 

 formed fragments and crystals of felspar ; 



(5) The high temperature resulting from this secondary heating, probably 



assisted by water holding alkaline salts and other mineral matter in 

 solution, appears to have further acted upon certain parts of the rock, 

 probably along joints or on the walls of small cavities, thus decomposing 

 and disintegrating the spherulites : such portions, on slow cooling, 

 assuming the structural character of a coarse-grained felsite, while also 

 possessing the mineral constitution of such a rock. 



It appears that, in this case, the changes are mainly molecular, 



