GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 487 



Underneath this again is a layer 45 cm. (18 ins.) thick of 

 fairly dark coarse grained rock rich in augite, containing 

 near the aplitic rock much biotite ; and beneath this is a 

 layer 30 cm. (1 ft.) thick of a coarse light-coloured essexite 

 rich in felspar and analcite, and containing patches of a 

 greenish decomposition product, and this graduates down- 

 wards into the main mass of coarse essexite. Exposed on 

 the same wall as specimen P, a few feet to the S.W., is a 

 lenticular mass of very coarse or " pegmatitic " rock, at 

 about 4*25 metres (14 feet) below the junction level. 



Megascopic Description. Dark, moderately fine grained 

 doleritic rock in which the constituent minerals are visible 

 to the unaided eye. 



Microscopic Description. Texture : Crystallinity, non- 

 crystalline; grainsize, medium, about 1 to 3 mm.; fabric, 

 hypidiomorphic granular to sub-ophitic. 



Minerals Present : 



Corrected for decomposition. By weight as measured. 



Felspar 



50% 



Felspar 44% 



Augite 



28 



Augite 18 



Iron Ores 



14 



Iron Ores 15 



Olivine 



4 



Serpentine 6 



Biotite 



2 



Ohlorites 6 



Apatite 



2 



Analcite 4 

 Stilbite and calcite 3 





100 



Biotite 2 

 Apatite 2 



100 



In making the correction for decomposition the stilbite 

 and calcite have been assumed to have been formed from 

 augite, and they have been taken as present in equal 

 quantity. 



The felspar is of two kinds, the more abundant being 

 plagioclase, the less abundant an alkali felspar — orthoclase, 



