158 R. A. WEARNE AND W. G. WOOLNOUGH. 



Olivine basalt. Summit of Mount Mitchell. — The rock is 

 hyalopilitic. The great bulk of it is. made up of minute 

 singly twinned felspar microlites with very perfect fluidal 

 arrangement. At first sight these appear to be sanidine, 

 as their extinction is almost straight, and there are no 

 albite lamellse visible. The refractive index however is 

 greater than that of cooked Canada balsam, so that the 

 mineral is oligoclase. There is very plentiful magnetite 

 in minute idiomorphic crystals. 



Much less abundant is augite in yellowish -grey subidio- 

 morphic grains, interstitial between the felspar laths. A 

 little ilmenite in very thin plates can be made out. 



There is quite abundant interstitial glass, brown to 

 brownish-green in colour and quite isotropic. 



Scattered small crystals of olivine up to 0*5 by 0*2 mm. 

 give the rock a porphyritic appearance on a small scale. 



A very few plagioclase crystals of the same order of size 

 also occur. Some of these are untwinned and look extremly 

 like nepheline, but yield a biaxial figure in convergent light. 



Some of the magnetite grains rise to porphyritic dimen- 

 sions. 



Rounded masses of small size of fibrous secondary 

 material occur, apparently natrolite. 



Porphyritic olivine basalt, Summit of Spicer's Peak. — 

 The rock has a pilotaxitic base of oligoclase microlites, 

 tiny octahedra of magnetite, needles of augite and small 

 pseudomorphs of serpentine after olivine. The arrange- 

 ment is strongly fluidal ; no glass is present. 



Scattered phenocrysts of acid labradorite up to 5 mm. 

 by 2 mm. occur. These are very clear and free from 

 decomposition and show perfect examples of Carlsbad, 

 albite and pericline twinning. They contain fairly abundant 

 inclusions of augite granules and long subparallel streaks 



