92 



and slaggy variety. Some of the iron-bearing constituents 

 have undergone oxidation, thereby imparting a reddish colour 

 to some portions of the rock. 



V. — Vesicular Vitrophyric Olivine Basalt. 



Rock 2. 

 Near the base of the outflow in Brown Lake. 



Macroscopic Characters. 



In hand specimens the rock is dark-brown to black in 

 colour, with a sub vitreous lustre and uneven fracture. It 

 shows strong evidences of having flowed, also that cooling 

 had taken place rather rapidly. In cross-section two layers 

 are seen, the external layer possessing a great many cracks 

 and fractures, whilst the internal, which had taken longer 

 to cool, is only slightly vesicular, having no cracks. It is 

 evident from this that the internal portion continued to flow 

 after the exterior layer had solidified, the strain being suffi- 

 ciently great to crack this hard layer transversely. The rock 

 is tachylytic in parts, containing a few small scattered pheno- 

 crysts of olivine. 



Microscojiic Characters. 



The rock has a distinct vitrophyric structure. The 

 groundmass is dark-brown in colour — in fact, some of the 

 sections were so dark that practically no light was transmit- 

 ted, except in the cases where phenocrysts occurred. The 

 glass is densely charged in places with magnetite dust in the 

 form of "cumulites," rendering those particular portions 

 almost opaque, whilst in other places only a few scattered 

 grains occur. It is highly vesicular, but the vesicules are ex- 

 ceedingly small. The microlites have a distinct fluxional 

 arrangement, being more or less parallel to one another. 



The microlitic components of the base are a few crystals 

 of augite and probably a few grains of olivine, with a very 

 small proportion of felspar. The augite is by far the most 

 abundant component in the glass, and generally idiomorphic. 

 The principal forms developed are sections parallel to the 

 (100), and more rarely those parallel to the (010) and (HI). 

 Several cruciform twins are to be seen besides the more com- 

 mon twin on the (100) plane. Small rosettes are also seen, in 

 which case sections parallel to the (100) are developed. 



The next component in abundance is the felspar. These 

 gave symmetrical extinctions up to 35°, and in all cases 

 proved to have the same characteristics as the plagioclase in 

 rock 1. 



