•506 MR. E. EXJTLEY ON [Aug. 1900, 



H 45 . Eotorua. — A grey to yellowish-white rock, with spheru- 

 litic structure, and showing in places a small amount of hyalite. 

 The rock has a cavernous or honeycombed aspect due to solfataric 

 action. 1 



Under the microscope this rock is seen to be an altered spheru- 

 litic obsidian, consisting almost entirely of spherulites of the large 

 brown microfelsitic type, similar to those met with at Mercury Bay, 

 except that these have undergone great change, while those of 

 Mercury Bay are practically unaltered. The whole rock, indeed, 

 appears to have been intensely affected by solfataric action. In 

 many places the spherulites are partly, in others almost wholly 

 bleached, the irregularly-shaped, brown, unaltered portions impart- 

 ing an almost brecciated appearance to the section, when viewed 

 in ordinary transmitted light. 



The alteration of these spherulites is apparently of the same 

 nature as that of the spherulites in the ' older rhyolite ' of Waihi 

 Beach (H 31 ) described on pp. 498-499 of this paper, but the 

 change does not appear to have been carried quite so far as in the 

 Waihi rock. The nature of the alteration seems to consist in the 

 gradual substitution of silica for portions of the spherulites which 

 have been dissolved : the process of molecular substitution having 

 apparently gone on with great exactitude, as in the silicification of 

 wood, the fibrous structure of the spherulites being beautifully pre- 

 served. (See PI. XXVII, fig. 5.) An interesting feature of the change 

 lies in the circumstance that, although the replacing substance was 

 opal-silica, the altered parts of the spherulites exhibit the same double 

 refraction between crossed nicols as those parts of the spherulites 

 which remain brown and unaltered. The dark crosses pass through 

 altered and unaltered parts of the spherulites without any appreciable 

 break or modification. It seems possible that the mere fact of 

 isotropic opal-silica replacing a fibrous structure suffices to impart 

 double refraction, no matter whether the replaced fibre was aniso- 

 tropic or not. Support is lent to this hypothesis by the feeble 

 double refraction shown by fibres of glass (* spun glass '), the extinc- 

 tion taking place in the direction parallel to the length of the fibre. 

 Again, in a section made from the white pulverulent crust on the 

 surface of a specimen of silicified wood (wood-opal) from Tasmania, 

 the fibrous structure of the woody tissue appears brilliantly illumi- 

 nated when the fibres are placed at 45° to the principal sections of 

 the crossed nicols, and becomes extinguished when these are turned 

 parallel and at right angles to the length of the fibres. It is clear, 

 therefore, that opal-silica, whether it replaces the fibres of woody 

 tissue or the fibres of a spherulite, develops the phenomenon of 

 double refraction. 



The spaces between the spherulites in this Eotorua rock are filled 

 with opal-silica. Where this has been deposited on the surface of 

 a spherulite, it has often assumed the condition of hyalite, exhibiting, 



1 I am indebted for this specimen to the kindness of Mr. A. Vaughan 

 Jennings, F.L.S., F.Gr.S., who procured it when visiting Eotorua in 1890. 



