OF MOUNT EREBUS, ANTARCTICA 109 



magnetite grains in a colourless glass. A number of black and brown opacite masses 

 suggest the original presence and subsequent disintegration of hornblende. 



Another fragment in this rock is a black vitrophyre containing phenocrysts of 

 oligoclase magnetite and augite in a black opaque matrix. 



No. X. Shonkinitic Kenyte Porphyry, erratic, Cape Royds. — This rock is a remark- 

 able kenyte in which the phenocrysts are those minerals which constitute the usual 

 constituents of teschenite and shonkinite, while the ground-mass is a normal trachyte. 



The phenocrysts are : 



(1) Labradorite felspar with extinction angle 30° corroded at the same time in the 

 centre and around the rim. 



(2) Bistre-coloured to purplish augite in small slightly corroded phenocrysts 

 sometimes surrounded by a corrosion rim of magnetite. 



(3) Large corroded phenocrysts of brown basalt hornblende with sapphire (?) 

 inclusions. In many cases the whole or a large part of the hornblende is pseudomorphosed 

 to magnetite, usually with a core of hypersthene. 



(4) Broken nepheline crystals and pseudomorphous aggregates of orthoclase and 

 sodalite after nepheline. 



(5) Groups of large idiomorphic soda orthoclase crystals. Veins consisting of an 

 intermixture of orthoclase and fluorspar traverse the rock. 



The ground-mass consists of a very fine-grained holocrystalline base of anorthoclase 

 aegirine -augite, riebeckite, and dusty magnetite in isometric grains and stunted rods. 



4. GROUP OF TRACHYDOLBRITES 



The name trachydolerite is most suitably retained for those rocks which have the 

 phenocrysts of porphyritic basalt or dolerite, and a ground-mass of a tephritic nature. 



Texture. — These rocks have porphyritic, megaporphyritic structure, and are usually 

 vesicular. The phenocrysts may be serial or hiatal. The ground-mass is a variable 

 from holocrystalline to glassy, with fabric ranging from pilotaxitic, doleritic, ophitic 

 or strahlenkornig to hyalopilitic or vitrophyric. 



Composition. — (1) Phenocrysts. — Olivine occurs in abundance as large corroded 

 crystals. Titaniferous augite, often enveloped by an outer zone of secondary 

 deposition, is also abundant and attains large size. Apatite and magnetite or ilmenite 

 occur as smaller phenocrysts, the former idiomorphic, the latter corroded. Enstatite 

 is not infrequent. Large felspar phenocrysts, ranging from oligoclase to basic 

 labradorite, may be present. 



(2) The Ground-mass. — The base consists of acid plagioclase, anorthoclase, 

 magnetite, purplish augite, more rarely green tegirine- augite, felspathoids, and glass. 



The characters which all these minerals present are so typical as to need no further 

 description. The order of consolidation is the normal one for dolerites. The pyroxene 

 may precede or follow the felspars. 



In hand-specimen these rocks all resemble porphyritic basalt. They are very dark, 

 almost black, in colour, and very rough in fracture from the unevenness of the grain size. 



J. 12(1921). Trachydolerite Breccia. Parasitic cone on Erebus. — Hand-specimen. — 

 Typical breccia with large and small fragments of lava. 



Micro- structure. — The section includes portions of both the fragments and the tuny- 

 base. 



Texture. — The inclusion is a porphyritic serial, vitrophyric trachydolerite, with a 



II R 



