Vol. 56.] ERUPTIVE ROCKS FROM STEW ZEALAND. 507 



in ordinary transmitted light, the mammillated surface, and, in 

 polarized light, the double refraction characteristic of that variety of 

 opal. Those parts of the section which lie between spherulites 

 frequently contain some silica which is, as a rule, perfectly isotropic, 

 save for a few bright doubly-refracting specks. In a few places in 

 the section, some nearly opaque matter occurs. It usually forms 

 thin irregular lines which have the appearance of lining cavities or 

 filling cracks. In reflected light this substance has a rather strong 

 pink colour, but concerning its nature or the pigment to which it 

 owes its colour it seems unsafe to speculate. The silica-percentage 

 of this rock is found by Mr. Philip Holland to be 75-90. 



H 46 . Eotorua. — Part of a dark-grey b o m b, ejected during the 

 eruption of Tarawera in 1886. 1 The form of this bomb was evi- 

 dently all but spherical and, when entire, it measured probably 

 between 2 and 3 inches in diameter. 



Prof. A. P. AY. Thomas, F.L.S., F.G.S., of University College, 

 Auckland, has so well described the rock of which these bombs 

 consist, that I cannot do better than quote his words : 



' Under the microscope thin slices of the rock show rather numerous small 

 crystals of felspar and yellowish augite. The felspars are mostly small and in 

 ledge-shaped sections, but a few rather larger ones are present ; they are nearly 

 all plagioclases, but there are a few distinct sanidines. The augite is in small 

 irregular crystals and granules. The ground mass of the rock is a grey glass 

 with abundant crystallites (globulites, longulites, and microliths) of translucent 

 substance, and grains of magnetite. Much of the augite should perhaps be 

 considered as belonging to the groundmass.' 2 



To this description there seems nothing to add. The section now 

 examined, although by no means so thin as could be wished, suffi- 

 ciently confirms all that Prof. Thomas says. He further adds : 



' Olivine was found in a few small crystals in some specimens, but as a rule 

 it is quite absent.' 



Discussing the precise character of the rock, he also says (he. cit.) : 



1 On account of the absence of olivine as an essential constituent, the presence 

 of abundant glass with microliths, and of a little sanidine, the rock has been 

 identified as an augite-andesite. Still, it must be admitted that the rock 

 approximates to the basalts, as is further shown by its chemical analysis . . . The 

 percentage of silica varies from 50*9 to 52"5, an amount which is lower than 

 that usually found in augite-andesites, whilst it is not higher than that found 

 in some basalts. But whether we call the rock an augite-andesite or a basalt 

 without olivine, does not affect the results stated above, that the lava of the late 

 eruption is basic, is new to the locality, and follows the acid rhyolites.' 



Prom the resemblance which the section of this bomb bears to 

 some of the Icelandic basalts without olivine, it seems probable that 

 Prof. Thomas is perfectly justified in regarding the rock as a basalt. 



1 For this specimen I am indebted to Mr. G. F. Rodwell, who visited the 

 scene of the eruption in 1887. 



2 'Report on the Eruption of Tarawera & Kotomahana, N.Z.' (Wellington 

 1888) p. 58. 



