500 ME. F. KUTLEY ON [Aug. I9OO, 



leading, in an extreme phase, to the obliteration of original structure 

 and the ultimate development of a felsitic condition. 



If such changes can be brought about in a rhyolitic lava of 

 Tertiary age, little wonder can be felt at the occurrence of like 

 changes in rhyolites of great geological antiquity. It even seems 

 possible, if not probable, that some of the felsites which now yield 

 no microscopic evidence that they were ever lavas, may originally 

 have possessed structures which, had they been preserved, would 

 have completely set at rest all doubt about their origin. 



H 32 . Waihi Monument ('older rhyolite , of Park). — A yel- 

 lowish rock with a spherulitic structure, and containing some small 

 dark-green plates which are easily scratched : these appear to be 

 chlorite pseudomorphous after biotite. The spherulites average 

 between -^ and J inch in diameter. 



Under the microscope, this appears to be a comparatively fresh 

 or very slightly altered representative of the preceding rock from 

 Waihi Beach (H 31 ). The spherulites, of which the rock is almost 

 entirely composed, are, however, quite as doubly refracting as those 

 that occur in the rocks of Omahu and Mercury Bay. 



Very small black crystals and specks are common throughout the 

 section. In most instances these may be regarded as magnetite. 

 In reflected light no distinct evidence of the presence of pyrites can 

 be detected. 



The spherulites are often packed closely together, but when 

 spaces exist between them, those spaces are filled with a colourless 

 substance containing numerous microlites, which in most cases are 

 colourless and give approximately straight extinctions. The colour- 

 less matter in which they lie is more or less isotropic, and mainly 

 consists of scales, apparently of tridymite, with possibly some micro- 

 felsite. 



The section contains a few porphyritic felspar-crystals, the largest 

 measuring over f, inch in length. They contain many glass-inclusions, 

 often show twin lamellae, and appear in many instances to be labra- 

 dorite. Some of them are much corroded. One of the largest 

 porphyritic crystals gives an extinction- angle of 38°, and the partial 

 interference-figure seen in convergent polarized light indicates the 

 oblique emergence of one of the optic axes just outside the field. 

 This may, therefore, be regarded as a section of anorthite parallel 

 to the brachypinacoid. The crystal is traversed by irregular cracks, 

 and contains many glass-inclusions. 



The rock may be called a rhyolite, or preferably an obsidian, 

 composed of microfelsitic spherulites with some porphyritic crystals 

 of felspar. 



H 33 . Waihi Beach. — A pale greenish-white rock with darker 

 green mottling, containing a few small, micaceous-looking plates, and 

 numerous little grains which have a vitreous lustre and occasionally 

 show conchoidal fracture. The specimen is so hard that the point 

 of a knife -blade barely makes any appreciable streak. 



