OF MOUNT EREBUS, ANTARCTICA 113 



Various irregularly shaped dark patches profusely rich in magnetite represent 

 either disintegrated xenoliths or the degradation products of basaltic hornblende. 



In J. 20 (1929), a very similar rock, the fine dusty magnetite of these dark areas 

 was clearly seen to be imbedded in a brownish isotropic matrix. The magnetite is clearly 

 of secondary origin, and it seems most likely that these rocks originally contained basalt 

 hornblende. 



J. 24 (1933). This is a microporphyritic serial basalt without olivine, the 

 phenocrysts being a basic labradorite. In this slide, too, occur black augite-opacite 

 aggregates secondary after hornblende. The base is intersertal, almost panidiomorphic, 

 hypocrystalline, consisting of felspar in needles, augite prisms, magnetite grains and 

 apatite in stout rods, with a trace of interstitial glass. 



The majority of the olivine less basalts have, it will be seen, the following general 

 characters : 



(a) Felspar, usually basic labradorite, forms 50 per cent, of the bulk or less. 



(b) Black patches crowded with dusty magnetite inclusions probably represent 

 disintegrated hornblende phenocrysts. 



(c) Olivine is only sparingly represented. 



((/) Apatite is less abundant than in the kenytes. 



(e) The glassy base consists largely of dark brownish-black glass undergoing 

 devitrification to magnetite, titaniferous pyroxene and hypersthene. It is full 

 of black magnetite grains both of primary and secondary origin. 

 (/) No felspathoids detected in this group. 



7. THE GROUP OF OLIVINE BASALTS 



The general characters of this group are so well known as to need no detailed 

 description. The peculiarities of the Antarctic olivine basalts examined by me seem 

 to be that they are usually coarsely porphyritic, and approach the limburgites on the 

 one hand and the basanites on the other. Felspathoid minerals are of common 

 occurrence as a minor constituent. The felspar of these rocks consists of basic 

 labradorite, and the pyroxene is a deep brown titaniferous augite. The olivine is 

 usually in large corroded phenocrysts. Felspar does not usually occur as phenocrysts, 

 but may do so in some types. The texture is usually holo- to hemi-crystalline, 

 porphyritic hiatal, dopatic or sempatic, megaphyric, and the fabric hyalopilitic. 



J. 59. — Porphyritic Felspar Olivine Basalt, Cape Koyds. — This rock is a black 

 porphyrite full of tear-shaped vesicles (drawn into that form by flow during the plastic 

 condition). 



The phenocrysts consist of basic labradorite, olivine, highly titaniferous augite, 

 magnetite, and apatite. The holocrystalline pilotaxitic base consists of acid 

 labradorite in laths, and augite and magnetite in idiomorphic grains. 



J. 28 (1937). Enstatite-olivine Basalt or Basanite , loose block, foot of Crater Hill. — 

 Texture. — Porphyritic serial, dopatic, mediiphyric. Ground-mass holocrystalline, inter- 

 sertal. 



Constituents. — Felspar, consisting of acid labradorite, occurs only as laths and 

 microlites of variable size belonging to the effusive period of crystallisation. The 

 olivine and enstatite form corroded microphenocrysts belonging to the intratelluric 



