20 ox HAUYNE-TRACHYTE AND ALLIED ROCKS. 



characteristically developed. In no other rock in Tas- 

 mania have ^ve seen the concentric zonal markings so 

 beautifully exhibited. Striped felspar is present in 

 quantity, its extinction angles being those of oligoclase- 

 andesine. We have not been able to measure an angle 

 high enough for albite on an 010 section, but a strip of 

 felspar, intergrown with a crystal of sanidine, gave an 

 angle of 20°, and this may be albite. The felspars are 

 uncommonly free from inclusions of the other minerals of 

 the rock. 



H(n'}}hh'}ul(>. — Next to felsj^ar, this is the most prominent 

 constituent in dark green columnar forms. The olive- 

 green color is often so deep as to make the mineral opaque, 

 and occasions difficulty in reading off the extinction angle. . 

 The absorption scheme is t ^ C -^ H, and the pleochroism 

 a yellowish-green, J) very dark green, t dark green, 

 sometimes opague. The extinction angle is unusually 

 high, the values which we obtained being 20°, 21°, 25°, 26°, 

 28°, 30°, ai°, 32°. These agree very well with Professor 

 Brogger's cataphoritic hornblende, though the absorption 

 scale does not correspond ; it is evidently a hornblende 

 with cataphoritic tendencies. 



Aug He is not frequent ; it occurs mostly in forms of the 

 prism. Extinction angle 38°, very pale green, non-pleo- 

 chroic : crystalline sphene, apatite and zircon are constant 

 accessories. The groundmass is not fluxional, but crystal- 

 line-granular. In it are a few rounded blebs of quartz, 

 aurrounded by a fringe of re-crystallised felspars, and 

 containing some moving bubbles ; magnetite grains in no 

 great quantity, and no mica discernable. 



Malchite. 



Sp.gr. 2-79. 



This rock was found on Mr. Cranny's property, adjoin- 

 ing Goad's farm, at Lymington. It occurs on the side of 

 the hill, but its geological relations were not further 

 examined. Locally it is called " basalt." 



Mam 'oscop ica 1 characters. 



Those of diorite, granular in texture, dark green in 

 colour, owing to the green hornblende which forms the 

 bulk of the rock. It is iron-stained along short irregular 

 cleavage planes. 



Mineral constituents. 



Hornblende, biotite, augite, plagioclase, apatite, sphene. 



