580 MINERALS AND VEINS .OF ^EGIRINE-SYENITE, 



i. The Parent Intrusive Mass op ^Egirine-Syknite. 



The syenite quarries of Bowral* have been somewhat largely 

 described in a previous communication! by T. G. Taylor and the 

 author; on that occasion, however, detailed attention was not 

 given to the small pegmatite and related veins so numerous and 

 well exposed by quarrying operations. 



The intrusive body, due to surface denudation, stands out as 

 an imposing mass, the "Gibraltar P^ock," rising nearly 1000ft. 

 above the neighbouring flat country, and is over half a mile in 

 width and about one and a half miles long. The intruded sedi- 

 mentary beds are Triassic shales and sandstones above, passing 

 evenly downwards into the Permo-Carboniferous productive coal 

 measures at a depth of less than 1000 ft. below. 



In dealing with the syenite, we had correlated it with bostonite 

 after a penological examination of the light-coloured variety 

 prevailing in the easternmost quarries. Since then, more detailed 

 work has revealed that originally the whole mass of the Gibraltar 

 syenite was fairly uniform in character, and that the "leucocratic" 

 variety owes its light colour to extreme carbonation of the ferri- 

 ferous minerals by the powerful action of highly carbonated 

 solutions, containing in addition fluorine, which circulated in the 

 vicinity of Loveridge's main quarry at the close of the pneuma- 

 tolitic stage. A little further to the south and east of this 

 locality, near the margin of the intrusion, the mephitic action 

 has been extreme and the felspars are completely kaolinised. 

 This is an example of deep-seated kaolinisation not infrequently 

 met with and difficult of explanation by normal agencies of 

 decomposition. 



In specimens of the less-altered rock obtained from the quarries, 

 the former sites of iron silicates are seen to be occupied by a 

 heterogeneous mass of decomposition products, chiefly carbonates. 



* Bowral is situated about 70 miles in a south-west direction from Sydney. 

 + " The Geology of Mittagong," Journ. Proc. Roy. Soc. N. S.Wales, 

 Vol.xxxvii, p. 306. 



