BY D. MAWSON. 



457 



found specimens representing a variety of stages in the devitri- 

 fication of the glass to the development of a porphyritic felsite. 



MacroscopicCharacter s. — Colour dark brown to black; 

 translucent in thin splinters. Fracture hackly; a well developed 

 perlitic structure causes it to break up into tiny granules a few 

 mms. in diameter. Texture glassy. Constituents chiefly dark 

 brown glass, in which occasionally the tiny white felspars are 

 evident. 



Microscopic Characters. — Texture hypohyaline por- 

 phyritic (vitrophyric). Minerals prese7it (in approximate propor- 

 tions by area) : — 



Glass 90% 



Felspai'-andesine (Ab^ Ang) ... ... 6% 



Hypersthene ^ 



Magnetite I 4% 



Apatite ) 



Glass base uniformly brown and isotropic; crossed by numerous- 

 perlitic cracks. Felspar's in clear idiomorphic crystals often 

 1 mm. long; generally rectangular in shape and exhibiting albite 

 twinning with the extinction angle of andesine. Regular 

 inclusions of a brown glass are invariably present; this character- 

 istic is well shown in the photograph (Plate xxii., fig.5). Corrosion 

 by resorption is always strongly evident, and in some of the less 

 glassy varieties of the andesite appears to have been followed by 

 further additions to the crystal of a more basic felspar. 

 Flypersthene in rods and rectangular prisms of a light green 

 colour, up to 0-5 mm. in length : pleochroism is very faint. 

 Magnetite in grains often attached to hypersthene crystals. It is 

 developed in about equal quantities with the hypersthene. 

 Apatite in long slender rods often crossing crystals of the other 

 minerals. 



Order of consolidation. — . 

 Apatite. 



Magnetite. 



Hypersthene. 



Felspar. 



Analysis (see appended table). 



