OF MOUNT EREBUS, ANTARCTICA 111 



The close relationship of the normal kenytes to the hornblende trachytes of 

 Observation Hill has already been referred to. The analyses of the Skuary kenytes 

 are of interest as being of rocks almost devoid of ferrous iron. In J. 5 (Table II, B, 

 p. 110), magnetite must be absent ; the dark grains resembling magnetite must be 

 riebeckite. J. 25 (Table II, C, p. 1 10) is also a riebeckite -bearing rock. The norm of 

 both these rocks (see Analyses IV and V, Tables V and VI, pp. 122, 123) indicates 

 presence of both nephelite and noselite molecules. The kenytes differ from the trachytes, 

 as regards chemical composition, in the following respects: they contain less silica, 

 more alumina, more lime, and more phosphoric acid. Zirconia is usually present. 



The rocks here designated plagioclase kenytes are of a much more basic nature. 

 Iron minerals are more plentiful, also magnesia. In alkali percentage as well as 

 general composition they correspond to the Tephrites and Basanites of Eosenbusch, 

 but the texture is so typically that of the kenytes that it is as well to regard them as 

 a basic kenyte. The norm gives their position in the American classification as being 

 in the subrang Salemose, which brings out relationships with camptonite from Maine, 

 nephelite basalt, dolerite and other rocks (cf. Chemical Analyses of Igneous Rocks, 

 by Washington, Professional Paper No. 14, U.S. Geol. Surv.). The basic inclusions 

 in tephritic trachyte, Forodada, Columbretes, are almost identical in chemical com- 

 position with the plagioclase kenyte (cf. Analysis 36, p. 355, Gesteinlehre, 2nd ed.). 

 This is additional evidence that this rock type is a basic differentiate of the kenytic 

 magma. 



5. THE GEOUP OF LEUCITOPHYEES AND TEPHEITES 



In hand-specimens these rocks resemble dark grey or reddish-brown basalts. 

 Vesicles are often arranged in a planophyric manner. 



Texture. — Porphyritic hiatal or glomeroporphyritic ; sempatic ; mediiphyric to 

 mediophyric. Vesicles abundant or absent: fabric of base, hypohyaline, pilotaxitic, 

 occasionally ocellar. 



Constituents. — The constituent minerals are those of the trachydolerites, but leucite 

 is relatively much more abundant, forming one of the chief constituents, and felspar is 

 less abundant. Leucite occurs as mediophyric and minophyric phenocrysts, which 

 are frequently altered to pseudoleucite. The other phenocrysts attain a smaller size. 



J. 57 (1499). Leucitophyre. — The chief crystalline constituents of this rock are 

 tabular oligoclase-andesine crystals, idiomorphic leucites and pseudoleucites, and 

 corroded prisms of pegirine-augite. The base consists mainly of red glass, which 

 contains fine microlites of felspar and segirine-augite. 



This rock is therefore an oligoclase leucitophyre. (Plate IV, fig. 6.) 

 The other leucite-bearing rocks are of a more basic character, and belong rather 

 to the group of tephritic basalts. 



J. 31. Leucite Tephrite. Loc. : Top of Crater Hill. — This is a reddish porous 

 rock, the vesicles of which are arranged in planes or bands. The core of the specimen 

 is dark basaltic -looking, and has small leucite and nosean phenocrysts of minophyric 

 size studded through the aphanitic base. The leucites are, under the microscope, seen 

 to be altered to a mosaic of orthoclase and nepheline (j>seudoleucite), and vary in size 

 from just megascopic to microscopic (minophyric to miniphyric). Their rounded 

 outlines are due to resorption. Other corroded microscopic phenocrysts are present 

 in ragged grains composed of olivine, light brownish hypersthene secondary after 

 hornblende, idiomorphic cumulophyres of uncorroded augite with hour-glass twinning, 

 and large corroded magnetite grains. 



