OF MOUNT EREBUS, ANTARCTICA 115 



Composition. — The phenocrysts consist of oligoclase, usually exceeding 5 mm. in 

 size, titaniferous augite 3 to 4 mm., smaller olivines and magnetite. Black opacite 

 pseudomorphous after hornblende also occurs (see Plate V, fig. 2), and are of mega- 

 porphyritic size. The base is hemicrystalline and consists of anorthoclase microlites, 

 brownish idiomorphic grains of titaniferous pyroxene, magnetite grains, and a yellow 

 devitrifying glass studded with inclusions. There are also colourless isotropic areas 

 studded with inclusions, and they have probably the composition of nosean or analcite. 



J. 36 (1944), Mount Bird. — This is described in the section dealing with Cape Bird 

 rocks (post, p. 125). 



Both the basalts rich in olivine and the limburgites described hereafter are similar 

 to the olivine basalts and limburgites described by David, Smeeth, and Schofield in 

 Proc. Roy. Soc. N.S.W., vol. xxix, 1895, and those described by Prior in the National 

 Antarctic Memoir. 



Chemical analysis shows that the basalts from Cape Barne and, in fact, all the 

 basaltic rocks of Ross Island differ from normal basalts and can be divided into two 

 classes, one of which has limburgite affinities, the other of which contains hornblende 

 or pseudohornblende phenocrysts and is related to the Kulaites (Table IV, p. 120). 

 The olivine basalt from Cape Barne, J. 4, is a typical member of the second group, and 

 one which does not display its affinities by pseudomorphs after hornblende. The 

 felspar molecules (see Table VI, p. 123) (high alumina) are sufficiently abundant to place 

 this rock in the Salfemane class, while the typical limburgitic basalt has a lower Al 2 3 

 percentage, and belongs to the class Dofemane. 



J. 10, J. 19, and J. 20 are basalts similar to those described by Prior from Sulphur 

 Cones, and his analysis (Table III, A, p. 114) gives their composition approximately. 

 Their relationship to Kulaite is evident both from mineral and chemical composition. 



The analysis of J. 4 corresponds closely with Prior's olivine basalt from the Gap 

 and his limburgitic basalt from Winter Quarters. 



The leucite tephrite, Crater Hill (see J. 31, also Analyses G, Table III, p. 114, and XI, 

 Tables V and VI, pp. 122 and 123), is allied to the limburgite J. 68 (see Analysis B, 

 Table III) on the one hand and to the basanites on the other. The amount of sulphuric 

 anhydride in the rock is very large, and shows that much of what was originally con- 

 sidered leucite is in reality nosean. The norm (see Table VI) differs remarkably from the 

 mode, especially in the development of felspathoid in place of felspar. 



The normative composition of the limburgite, J. 68 (Table VI, No. IX, p. 123) bears 

 out the microscopic determination of the remarkable pseudomorphous aggregates after 

 felspar. It is a typical Kossweinose, as a comparison with the analysis of the Rosswein 

 type rock (Table III, C, p. 114) shows. 



8. GROUP OF THE LIMBURGITES 



In this group felspar is absent, or only subordinate in amount. They closely 

 resemble the porphyritic basalts in hand-specimen, and are usually coarsely 

 porphyritic in olivine and augite. The pyroxene is generally of two generations. The 

 order of consolidation was normal. 



The usual texture is porphyritic hiatal, sempatic, magnophyric with intersertal 

 base, which may be hyalopilitic, diabasic, or strahlenkornig (divergent radial). 



J. 60 (523 ?). Limburgitic Basalt. — Texture. — Porphyritic hiatal, dopatic, magno- 

 phyric ; ground-mass diabase grained. 



