OF MOUNT EREBUS, ANTARCTICA 103 



extinction due to strain, and Carlsbad, Baveno, and albite and Manebacb twinning ; 

 pyroxene undergoing decomposition to ferrite and serpentine; magnetite and 

 ilmenite ; and much colourless isotropic glass (probably of analcite or nosean com- 

 position), and also a dark glass. The structure may be described as microporphyritic, 

 sempatic to persemic in prismatic and equant phenocrysts. 



J. 16 (1925). From the same locality. — This is a holocrystalline, microporphyritic- 

 serial, persemic rock with pilotaxitic fabric. It consists of lath-shaped anorthoclase, 

 with oligoclase, jegirine-augite, magnetite, and ferritic decomposition products. 



J. 15 (1924). Also from Inaccessible Island. — This rock is very similarly 

 constituted, but contains a few megaphyric corroded olivine crystals. The texture 

 is hypocrystalline trachytic. The minerals are anorthoclase, oligoclase, olivine, and 

 pegirine-augite, with a clear colourless isotropic base probably of felspathoid 

 composition. 



The trachytes from Inaccessible Island have in hand-specimen the appearance of 

 aphanitic andesite, and in mineralogical composition too they form a transition phase 

 between the trachyphonolites and augite andesites. 



Remarks. — The felspars in all this miscellaneous series of trachyte greatly pre- 

 dominate in amount, forming always 80 per cent, or more of the total. Olivine is 

 rare or absent ; magnetite is not common ; pegirine is the chief dark constituent. 



It is of particular interest that two types of inclusions dominate in the trachyte, 

 viz. sanidinite (orthophyre) in the phonolitic and orthoclase aegirine trachytes and 

 camptonite in the oligoclase trachyte. 



THE TRACHYDOLERITE AND KENYTE GROUP 



The kenytes are usually considered to be a facies of trachydolerite. For the rocks of 

 the Erebus series I prefer to create three groups of the trachydolerite family, viz. the 

 Acid Kenytes, the Basic Kenytes, and the Trachydolerites proper, to be distinguished 

 by the nature of the phenocrysts and ground-mass, thus : 



Kenytes (Acid and Basic). Phenocrysts, anorthoclase (almond-shaped), aegirine. 

 Acid Kenytes. Phenocrysts of type ; ground-mass, structure and composition 



of alkaline trachyte. 

 Basic Kenytes. Phenocrysts of type, also tabular oligoclase ; ground-mass, 



structure and composition as in basic phonolites and tephrites. 

 Trachydolerites. Phenocrysts, titaniferous augite and olivine as in dolerite ; 



ground-mass, structure and composition of tephritic and basanitic basalt. 



The kenytes are all rough-looking rocks, varying from light grey to black in colour, 

 and usually markedly porphyritic in plate- like or almond-shaped felspars. They are 

 conveniently divided into two types : the acid type, obtained from the Skuary, Mount 

 Erebus, Cape Royds erratics, and Cape Barne; and the basic type, derived typically 

 from Turk's Head, Mount Erebus, and Cape Royds erratics. 



Composition of Kenytes. — The phenocrysts of the first generation consist of felspar, 

 alkaline pyroxene, and more sparingly magnetite and olivine. Rarely leucite 

 occurs as a product of intratelluric crystallisation. The felspar phenocrysts of 

 the typical acid kenytes are almond-shaped anorthoclases (as in rhombenporphyr) , 

 and of the basic type rounded oligoclases of tabular habit. Both felspars contain 

 olivine, pyroxene, and magnetite inclusions. 



