GEOLOGY OF THE JENOLAN CAVES DISTRICT. 6i i 



some of the calcite patches from their general appearance 

 appear to represent complete pseudomorphs after that 

 mineral. As a secondary product of a ferro-magnesian 

 mineral it is often associated with the other alteration 

 products, chlorite and iron ore. 



Epidote of a yellowish-green colour is occasionally 

 noticed as an alteration product of the phenocrysts and 

 grouudmass. It is rather plentiful in slide x 65, which 

 shows typical replacement of felspar. 



The other secondary products, kaolin and sericitic- 

 material as already indicated, represent the alteration of 

 the felspar. 



The grouudmass is microcrystalline, of a brownish- green 

 colour. It is much obscured by alteration, and inclined to 

 opacity, and does not admit of any detailed description. It 

 is generally composed of felspar and its decomposition pro- 

 ducts, intimately mixed with a finely fibrous chloritic 

 mineral. Wherever it penetrates the quartz and felspar 

 phenocrysts it is of a brighter green, and more transparent, 

 and illustrates well the alteration of the matrix into chlorite 

 by infiltration. From the result of the analyses the con- 

 clusion is drawn that the felspar is essentially plagioclase, 

 as the low percentage of potash would not allow much 

 room for the presence of orthoclase, the potash present 

 constituting orthoclase as phenocrysts. Microscopic quartz 

 is present in small amount only. Oalcite is scattered 

 through it plentifully in patches and microscopic aggregates. 

 A few inclusions of small fragments of other rocks were 

 noticed, principally of sedimentary origin, viz., slate etc., 

 also of a finer-grained igneous rock of a cognate type. 



One of the features of this rock is the high proportion of 

 phenocrysts to groundmass. This is apparent in both hand 

 specimen and section, giving to the former a phaneo- 

 crystalline appearance at first sight. The phenocrysts 

 represent fully 75 per cent, of the rock by volume. 



