websterite of Williams The diallage is made up of lamellse, 

 orientating optically in different stripes, and the crystals have 

 sometimes been mechanically beat into deformed curves. The 

 enstatite is in squarer forms, much serpentinised and in parts 

 altered into bastite. 



13. Enstatite Porphyrite. 



Sp. Gr. 2-84. 



Min. Const.: Chloritic pseudomorphs after enstatite, augite, fel- 

 spar in a ground mass of felspar laths and quartz, 

 magnetite in vesicles, quartz, calcite, epidote. 



Macroscopical Structure. In general appearance this is a 

 dull, greenish-grey coloured and finely granular rock, 

 with numerous distinct but minute pittings of a darker, 

 almost black, shade scattered throughout ; these are 

 apparently the chloritic pseudomorphs after the pyroxenic 

 constituents. These markings are, although usually plenti- 

 ful, more pronounced in some samples than others. The 

 most noticeable feature in this rock is its highly vesicular 

 character throughout. The vesicles vary from small cavities, 

 which are scarcely discernible to the naked eye, to others 

 which reach a comparatively large size, occasionally 

 measuring from two to three inches in diameter, They are 

 almost invariably lined with a thin coating of colourless 

 shining secondary calcite, many of the larger ones being 

 quite filled with this substance. There is sometimes a rather 

 large quantity of dark coloured epidote in fibrous fan-shaped 

 aggregations in close association with the calcite. This 

 porphyrite is fairly hard and tough, breaking with a hackly 

 fracture. The exposed surfaces are usually weathered to a 

 pale rusty brown. It occurs in the form of an intrusive 

 dyke, which is apparently about two chains in width, as seen 

 exposed by the road cuttings near the 11-Mile Peg on the 

 Waratah-Corinna road. In the near vicinity are rocks of 

 sedimentary origin, but now much altered, and mainly 

 represented by cherty quartz. 



Microscopical Structure. — Nearly all the phenocrysts have 

 been converted into chlorite of a pale greenish hue, slightly 

 pleochroic, and giving a blue slate interference colour. F rom 

 their forms we judge the original minerals to have been, 

 in their order of frequency, enstatite, augite, felspar. In one 

 instance one of the porphyritic felspars has resisted the 

 process of change. Its extinction angle is moderate : it is 

 probably labradorite. The enstatite crystals are entirely 

 idiomorphic, of prismatic habit and often cross jointed. 

 From these tranverse fissures and from the margins the 



