GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 537 



the mineral, is common in the pegmatites as in the main 

 mass. A photograph of one of the large plagioclase crystals 

 between crossed nicols is shown in Plate XXXIX, Fig. 5, 

 which shows the analcite plainly. There is also seen the 

 carlsbad twin-plane in the upper part of the picture, and 

 in the lower part faintly the kaolinised outer zone of alkali 

 felspar. The analcite veins are very clearly seen by ordinary 

 light if the condenser be sufficiently stopped down. 



Pyroxenes. 



There are three varieties of pyroxene present in the 

 pegmatites : — 



(1) A pale green non-pleochroic diopside in nearly idio- 



morphic stout prisms, some, but not all, having an 

 outer zone of aegyrine-augite. 



(2) A light purplish-brown feebly pleochroic augite with 



hour-glass structure, chiefly in large prisms reaching 

 occasionally 3 cm. in length, but partly also interstitial 

 to felspar, and generally similar to the augite of the 

 main rock. It is never bounded by segyrine-augite. 



(3) yEgyrine-augite, pleochroic from dark grass-green and 



olive-green to yellow, irregularly distributed as small 

 imperfect prisms and as a narrow outer zone to the 

 diopside crystals. 



One instance only was observed of intergrowth of the 

 brown augite and the diopside, the latter occurring as an 

 irregular outer zone. In relative abundance the brown 

 augite is predominant, and the aegyrine-augite quite sub- 

 ordinate, forming less than one per cent, of the rock. 



Other Minerals. 



The other minerals are all similar to those occurring in 

 the main rock. The biotite is of the usual strongly pleo- 

 chroic brown variety, the crystals averaging about 1 mm. 

 in diameter. The ilmenite presents a triangular skeleton 



