OF MOUNT EREBUS, ANTARCTICA 117 



J. 34 (1942). Magma Basalt, Tent Island. — Hand-specimen. — Black and vesicular. 



Texture. — Porpkyritic kiatal, dopatic, dominantly magnipkyric, tkougk a few 

 mediopkyric phenocrysts occur. Fabric vitropkyric. 



Constituents. — Pkenocrysts — zoned, corroded oligoclase and labradorite felspar 

 filled with apatite and glass inclusions, ligkt greenish-brown a?girine- augite 

 (titaniferous), corroded olivine, sparing apatite rods. Tkese pkenocrysts are imbedded 

 in a black opaque mass wkick kas by reflected ligkt tke steel-grey colour of magnetite, 

 and probably consists mainly of iron ore. (See Plate TV, fig. 5.) 



J. 35 (1943), from Gap Moraine, Hut Point. — Tkis contains corroded pkenocrysts 

 of enstatite and olivine and no otker pkenocrystalline mineral. Tke base consists of 

 a brown to black mass, containing plagioclase microlites with featkery and pronged 

 ends (keraunoids and swallow- tailed microlites), imbedded in acryptocrystalline matrix 

 of black magnetite, brown, almost isotropic, pyroxene crystallites and brown pyroxenic 

 glass. In texture tkis rock is a porpkyritic kiatal, sempatic basalt vitropkyre. 



J. 17 (1926), Tent Island. — Tkis is a red vesicular lava, and contains pkenocrysts 

 of oligoclase- andesine and of greenisk-brown augite. Tke former are corroded and 

 zoned, wkile tke latter are idiomorpkic in rods. Corroded olivines are also present. 

 Tkese pkenocrysts are imbedded in a reddisk-brown, opaque, glassy base, consisting 

 largely of iron ores. 



J. 22 (1931). Magnetite Basalt, Cape Barne. — Hand-specimen, dark compact bluisk- 

 black rock, witk small vesicles and slightly porpkyritic in felspar. 



Texture. — Porpkyritic kiatal, vesicular, dopatic, mediopkyric to micropkyric, witk 

 vitropkyric fabric in base. 



Constituents. — Tke felspar pkenocrysts consist of basic labradorite twinned on 

 Carlsbad, Baveno, and Maneback laws, as well as on tke albite and pericline laws, several 

 types of twinning occurring togetker in eack crystal. Tkis felspar occurs sparingly 

 as megascopic, but plentifully as microscopic pkenocrysts. Small augite rods forming 

 interpenetrating twins and corroded olivines also occur as microscopic pkenocrysts. 

 Tke base is made up mainly of magnetite and dark isotropic glass. 



Tke most remarkable tking about tke magnetite basalts is tke reversal of tke order 

 of consolidation, tke magnetite kaving been one of tke last minerals to form. 



GENERAL DISCUSSION ON DIFFERENTIATION 



One of tke most striking points about tke Antarctic lavas examined is tke kigkly 

 differentiated nature of tke effusive products. Tke abundance of limburgites, 

 magnetite basalts, olivine nodules, sanidinite inclusions, etc., and tke occurrence of 

 basalts witkout olivine, felspar basalts and olivine basalts in different interbedded 

 flows, skow tkat differentiation was very complete, and that the eruptions tapped 

 now one, now another portion of tke magma. 



Specific gravity played a most important part in tke differentiation. Tkis is skown 

 by tke occurrence of felspar basalts and augite andesites wkick R. A. Daly kas 

 demonstrated to be formed by gravity differentiation (Jour, of Geol., July-August 1908). 

 Tke formation of tke magnetite basalt is probably due to gravity; just as magnetite 

 kas in one specimen, P. 60, formed a secondary deposit round felspar pkenocrysts, so kave 

 pkenocrysts of various minerals become enveloped in a clump of magnetite and dragged 

 down by tke weigkt of tke secondary deposit into tke keavier magnetite layer. In tkis 

 way tke pkenocrysts of tke magnetite basalts probably originated. 



n s 



