OF MOUNT EREBUS, ANTARCTICA 125 



the limburgites, and the tephrites. The Kulaite of Kula, described by Washington, 

 agree with them in its characters, so that this name has been chosen for them. (For 

 Analysis see Table IV, p. 120.) 



Closely allied to this group of rocks, but deficient in the hornblende and its pseudo- 

 morphs, are the tephritic and basanitic dolerites of the next group. 



From the close similarity of the two groups it would seem that the kulaites are 

 derived by a mixture of a camptonitic with an alkaline doleritic magma. 



THE CAPE BIED DOLERITES 



P. 334 (654). Olivine Basanite, Cape Bird. — This is a compact, dark porphyritic 

 rock, megapkyric in olivine. It has a porphyritic hiatal structure, and its base is very 

 fine-grained, hypocrystallme and hyalopilitic in fabric. 



Olivine occurs as broken and corroded phenocrysts. Felspar occurs sparingly as 

 phcnocrysts, also of broken and very corroded outlines. The felspar phenocrysts 

 consist of albite. In addition, we have rounded and corroded nepheline phenocrysts 

 and patchy masses of cancrinite after nepheline. 



The base consists of pyroxene, magnetite, lath-shaped felspars, and glass. 



The larger pyroxene crystals of the base are idiomorphic and consist of a brownish 

 to greenish pleochroic aegirine-augite. 



This rock is a very alkaline type of dolerite. The low felspar percentage puts the 

 rock in this basic division, but it has strong affinities with the tinguaites and phonolites. 



P. 327 (649). Alkaline Dolerite. — This rock has close affinities with the kulaites 

 but is devoid of the basalt hornblende and its alteration product. 



The structure is porphyritic, serial, sempatic, minophyric, with a tendency to a flow 

 arrangement. The texture is hypocrystallme with a variety of pilotaxitic fabric. 



The main phenocrystalline constituents : augite, consisting of a faintly pleochroic 

 greenish to brownish pyroxene as in the kulaites ; zoned and corroded plagioclase, ranging 

 from oligoclase to acid labradorite ; and magnetite. 



The base consists of brownish augite, magnetite, ilmenite, plagioclase, and a fels- 

 pathoid (probably leucite), and glass. 



In both texture and composition this rock exhibits a great divergence from normal 

 dolerites and an approach to the basic kenytes and kulaites. Felspar and pyroxene 

 are about equal in amount. 



P. 322 (642). Limburgitic Dolerite. — This rock is the most basic rock examined 

 for this locality. It is coarsely porphyritic — serial, in olivine ranging from magnophyric 

 size downward, and in enstatite ranging of minophyric size. These phenocrysts 

 are always broken and corroded, and sometimes invaded by the ground-mass, and clearly 

 belong to the intratelluric period. The first augite of the effusive period was only 

 faintly titaniferous and forms light-brownish idiomorphic crystals, but the finer augites 

 are deep in colour, being more highly titaniferous. Broken magnetite phenocrysts 

 occur, but, in addition, there is second generation of magnetite in the base. The felspar 

 percentage is very small and this mineral occurs only as fine microscopic needles of 

 labradorite. A small amount of interstitial glass occurs in the base. The order of 

 consolidation was : (1) magnetite, (2) olivine, (3) enstatite, (4) augite, (5) magnetite, 

 (6) felspar, (7) glass. 



The most remarkable feature of the three basic rocks described is the dearth of 

 apatite, which is usually an abundant mineral in basic rocks. This feature clearly links 



II T 



