GEOLOGY OF THE JENOLAN CAVES DISTRICT. 



381 



Cede it e etc. is plentiful, occurring as grey patches up to 

 1 mm. in size and also scattered microscopically through 

 the groundmass. It shows strong absorption and rarely 

 traces of cleavage. It is an alteration product of the 

 felspar, both phenocrysts and in groundmass. The amount 

 present in this rock is high, as indicated by the presence of 

 2*83% of 00 2 in the analysis. No doubt magnesite and 

 possibly dolomite are present, as it is not at all likely that 

 the lime only is combined with the 00 2 . The appearance 

 of the carbonate mineral in section suggests this also. The 

 carbonates are due in part to infiltration and not entirely 

 the result of the alteration in situ of lime-bearing minerals. 



The groundmass is cryptocrystalline mainly, to micro- 

 crystalline in texture, and is generally distinctly felsitic in 

 character. It is of a grey colour and somewhat altered to 

 a kaolinised product. Where determinable, it is seen to be 

 composed of a microscopic mixture of orthociase and quartz, 

 the former predominating. Plow structure is well marked 

 in places. Calcite is plentifully scattered through it in 

 large patches and also in microscopic aggregates. A little 

 sericite has also developed mainly along lines of flow. Some 

 sections show complete silicification of the groundmass to 

 a microcrystalline granular quartz. 







Chemical Composition. 









Per cent. 





Per cent. 



Si0 2 ... 



.. 



70-40 



co 2 



2-83 



A1 2 3 





13-13 



Ti0 2 



0-30 



Fe 2 3 





1-25 



Zr0 2 



absent 



FeO ... 





1-75 



P 2 5 



0-08 



MgO ... 





0-64 



so 3 



* trace 



CaO ... 





2-74 



CI 



trace* 



Na 2 



.. 



. 3-47 



S (FeS 2 ) ... 



absent 



K 2 ... 





. 2-22 



Cr 2 O s 



absent 



H 2 O(100° 



o.) 



0-15 



NiO and CoO 



absent 



H 2 O(100° 



C.-i 



-) 1-35 



MnO 



0-04 



