Yol. 55.] EKTOLITES OF THE HAUEAKI GOLDFIELDS. 457 



that a similar weak double refraction, apparently due to tension/ 

 may be seen in certain bands in obsidian. The magma in 

 which the microfelsite-spherulites were formed was filled with 

 streams of trichites, and the fluxion-structure which they denote 

 passed without interruption through the spherulites, the sub- 

 sequently-developed arms of which produced no perceptible dis- 

 turbance of the trichites. The later stage, consisting in the deve- 

 lopment of the small ordinary spherulites, the reheating of the 

 rock, its eff'ect upon the surfaces of the microfelsite-spherulites, and 

 the reduction again of already solidified portions of the lava to a 

 state of fusion, followed by a final complete solidification, are points 

 which have already been admirably dealt with by the late Hermann 

 Vogelsang, in his description of the Tolcsva rhyolite. It would 

 be superfluous, therefore, to do more than allude to his masterly 

 examination of a rock which so closely resembles the one now 

 under discussion. 



The proof of the secondary fusion rests, according to Vogelsang, 

 upon the rounding of the arms of the large spherulites and upon the 

 indication by trichites of a decidedly fluid movement in the rock 

 immediately adjacent. These points can all be verified in the rock 

 from Omahu, in which, furthermore, the arms of the brown sphe- 

 rulites often show a distinct isotropic border, fully attesting the 

 superficial vitrification of these elsewhere partially devitrified bodies : 

 no more ample proof of this secondary fusion could be required. A 

 leading feature in this Omahu specimen is the presence of lithophysse. 

 A portion of one of them is included in the thin section of this rock. 

 It shows several irregular cavities bordered by reddish-brown, nearly 

 isotropic matter,, and tridymite. 



Hg [176]. Waikino, near Waihi. — This is a small fragment 

 of a pale bluish-grey rock, containing little angular pieces of pumice 

 of a somewhat darker colour. These included specks and fragments 

 have a distinctly vitreous lustre. The rock is rather harsh to the 

 touch. Under the microscope the section is seen to be largely com- 

 posed of fragments of pumice, embedded in a glassy matrix slightly 

 devitrified by globulites. The devitrification is, however, only 

 incipient, and both the fragments and matrix are almost completely 

 isotropic. The amount of matrix is so small that the rock consists 

 almost exclusively ot pumice-fragments. The section contains some 

 corroded fragments of felspar-crystals, which appear to represent 

 oligoclase and andesine, and a very few rounded fragments of 

 devitrified rock of cryptocrystalline character — ^in one instance with 

 brown, more or less glassy matter, which ma}- be part of a micro- 

 fclsite-spherulite. The rock is essentially a pumice -tuff. (PI. 

 XXXIV, fig. 6.) 



Hg [178j. Waikino, near Waihi. — A pale greyish-white, 

 lithoidal rock, with a brownish banding, the specimen glistening 



^ ' Microsc. Char, of Yitr. E.ocks of Montana,' Quart. Journ. Greol. Soe. 

 vol. xxxvii (1881) p. '6^3,. 



