The Bocks of Tristan d'Acunha. 45 



The augite is in large crystals often intergrown with magnetite, 

 and enclosing small granules of olivine ; it frequently shows twinning 

 and zonal structure. 



The felspars are large individuals, having the vitreous look of 

 sanidine ; they are twinned on the Carlsbad and Baveno systems, 

 though the bigger crystals are often unstriated, they are probably 

 labradorite from measurement of the angles of extinction. The 

 lath-shaped striated crystals sometimes are aggregated in radiating 

 bunches. 



The olivine is rare, with little brown alteration products round the 

 outside ; it is often adpressed against larger crystals of magnetite, 

 and is included as granules in the large augites. 



The matrix is composed of felspar laths, augite, enstatite, 

 magnetite, and glass. 



The felspar laths are very frail things, showing incomplete growth, 

 and seldom show a definite termination. The larger forms include a 

 quantity of the matrix. 



The augite is in small, ill-formed, light wine-red laths, giving 

 oblique extinction, and is not so strongly refractive as the enstatite, 

 with which it is often intergrown. The crystal forms tend to the 

 simple prism and basal plane, and the minute crystal laths often 

 show the hour-glass structure. 



The enstatite is in more refractive laths, giving straight ex- 

 tinction and rhombic sections, and is not so plentiful as the augite ; 

 it is slightly coloured yellow. Down the centre there is the 

 characteristic tubular space filled in with glass. 



The magnetite is in crystals and granules. 



The glass is colourless, with minute colourless refractive hair-like 

 growths. An isotropic mineral, sometimes showing hexagonal 

 sections, at others irregular, is probably one of the felspathoids, or 

 their alteration product, analcime ; the great rarity of the mineral 

 makes it impossible to determine it ; it contains the same hair-like 

 crystals as the glass. 



Slide A fine-grained, creamy-coloured rock, resembling a tuff 



t" h t 9 * n a PP earance > with rounded patches of coarser materials ; 



in a similar but harder sample, large sanidine can be 



seen with the naked eye. This loose tuff has many points of 



similarity with the black basaltic-looking rock from Nightingale 



Island (Slide No. 1152). 



The matrix is a felspathic paste, on which there are ill-formed 

 laths and dusty granules of exceedingly minute size. The laths are 

 incipient felspar, while the bright yellow microlites and granules 



