IG ON HAUYNE-TRACHYTH AND ALLIED ROCKS. 



have preserved their characteristic contours. The ground-- 

 mass is much decomposed, but seems to consist essentially 

 of small sanidines. Numerous black needles obscured by 

 ferrite may represent acmite or aegirine. 



Aegirine-Trachyte. 



Sp. gr. 2 "61. 



There are two or three varieties of trachyte, with needles 

 of the soda-pyroxene aegirine entering largely into the 

 composition of the groundmass. The most striking of 

 these is a greenish rock, markedly porphyritic and fissile 

 by reason of parallel layers of tabular sanidine crystals,., 

 found on the beach at Port Cygnet south of the Regatta 

 Ground. The only other porphyritic mineral is augite. 

 The plates of sanidine lie preponderatingiy in one direction 

 in layers, giving rise to divisional planes, along which the 

 rock cleaves more easily than in a direction perpendicular 

 thereto. 



Microscoj)ical characters. 



Inclusions of minute needles of augite (or aegirine) are 

 frequently arranged in zonal form round the periphery of 

 the sanidines, and the margins of the large felspars often 

 melt imperceptibly into the groundmass, the magma of 

 which has apparently corroded them. A crop of microlites 

 is usual along these imperfect edges. The sanidines are 

 clear, and enclose crystals and fragments of augite, besides 

 indefinable microlites and glass inclusions. 



Augite. — Sections in the zone of 001 and 100 are 

 common. 



The colour of these porphyritic pyroxenes is a rather 

 deep green ; they are distinctly pleochroic. The extinc- 

 tion angles are very variable, and the character of extinc- 

 tion is undulose, probably in consequence of mixtures of 

 normal and soda pyroxene. The extinction of the central 

 portion of a crystal will be 38°, while that of the margin 

 will be straight or nearly so. Sometimes a crystal is found 

 extinguishing at about 5° or 6° in one direction, with a 

 pale yellowish fringe extinguishing at the same angle in 

 the opposite direction. Inserting the quartz wedge with 

 its axis of least elasticity covering the elasticity axis of 

 the pyroxene nearest to the vertical crystallographic axis 

 of the latter, we notice that the colour falls till it is 

 replaced by darkness. In petrographical language, com- 

 pensation has set in. By this we know that the axis of 

 elasticity in the two crystals (the quartz and the augite) 

 are dissimilar. As the direction in question is that of 

 least elasticity in the quartz, it follows that it is that of 



