496 ME. P. RUTLET ON [Aug. I9OO, 



which, in convergent polarized light, gives a partial biaxial inter- 

 ference-figure of positive sign. 



H 27 . Waitekauri. — A bluish-grey vitreous rock, much re- 

 sembling some of the perlitic rocks of Schemnitz in Hungary, 

 Under the microscope there is no apparent fluxion-banding, and (with 

 the exception of eyelash-like trichites) the glassy matter is almost 

 wholly free from microlites ; but numerous porphyritic crystals of 

 felspar, mostly sanidine or oligoclase, quartz often corroded, biotite, 

 and occasional small specks of magnetite, are present. 



The perlicity of this section is extremely well-defined, and is 

 remarkable as serving, in certain spots, to illustrate the transition 

 from straight to curvilinear cracks. As pointed out many years 

 ago, 1 the curvilinear perlitic cracks are almost invariably packed 

 between straight or approximately straight fissures, and are not 

 traversed by the latter. 



In parts of this section cracks may be seen spanning transversely 

 the areas between rudely parallel and comparatively straight fissures. 

 These transverse cracks are, in some cases, almost straight, while 

 in other instances they show a distinct tendency to describe perlitic 

 curves. When this is the case, the perlitic bodies are elongated 

 between the rungs of the ladder-like systems of cracks, as shown in 

 PI. XXVII, fig. 2. Such a scalariform perlicity has, so far as 

 I am aware, not hitherto been noticed. 



The rock, apart from its porphyritic crystals and its fissures, is 

 remarkably isotropic. 



Mr. Philip Holland has kindly estimated the silica in this rock. 

 It amounts to 73*45 per cent. This sets at rest any doubt about 

 the rock being an obsidian, the Hungarian perlitic rocks, which 

 this somewhat resembles, being in many cases hyalodacites, which 

 are characterized by a lower percentage of silica. 



H 2 ^. Waitekauri. — Apparently a breccia composed of dark -grey, 

 yellowish-brown, and white fragments. The rock is generally very 

 dark, even black in parts, with a granulated appearance and with 

 small dark specks which have a vitreous lustre, the specimen slightly 

 resembling a fine-grained schorlaceous rock. 



Under the microscope the smaller fragments are seen to be 

 felspar, mostly andesine and labradorite. There are often, however, 

 crystals of sanidine, some of them twinned on the Carlsbad type 

 and, in one instance, on the Baveno type. Occasionally the plagio- 

 clastic felspars show zonal banding, the extinction-angles indicating 

 that the outer zone is andesine, while the inner portion is labra- 

 dorite. Crystals of augite and of a rhombic pyroxene, as well as 

 pyrites, occur in this section, together with fragments of andesite 

 and rhyolite, the latter sometimes spherulitic. These spherulites 

 are of the brown, microfelsitic kind. All these fragments of rocks 



1 ■ On some Structures in Obsidian, Perlite, & Leucite ' Monthly Microsc. 

 Journ. vol. xv (1876) pp. 179 et seqq. & pi. cxxxiv. 



