GEOLOGY OF THE JENOLAN CAVES DISTRICT. 375 



idiomorpkic, but a greater number of crystals are irregular 

 or rounded in shape. It shows the typical method of cor- 

 rosion along boundaries and lines of weakness as cleavage 

 planes and cracks, instead of the pocket-like intrusions of 

 of the groundmass, as in the case of the quartz. Plagioclase 

 predominates, but the multiple twinning is too obscured by 

 alteration to admit of the measurement of extinctions 

 except here and there. The results of several determina- 

 tions place it mainly as andesine, approaching in some to 

 oligoclase-andesine. Orthoclase seems to be fairly plentiful, 

 but this conflicts with the percentage of potash returned 

 in the analysis, so a second determination of the alkalis was 

 undertaken, on a sample secured recently at a different 

 spot to that upon which the analyses were made, the 

 material for which was collected some time previously: 



Recent sample. Earlier sample. 



Potash 0*56% 0*88% (analysis) 



The quantity of orthoclase present in this rock must con- 

 sequently be small. It is therefore probable that what is 

 assumed to be orthoclase in section represents plagioclase 

 twinned on the Carlsbad and albite laws, the finer lamellae 

 of the latter being obliterated by alteration, while the 

 more prominent Carlsbad type is still noticeable. 



The felspar is never fresh, but has undergone more or 

 less alteration into kaolin, sericite, chlorite, calcite and 

 epidote. These secondary products occur in abundance 

 in the order named. The amount of alteration varies in 

 different sections, some showing almost complete change 

 to the above products. 



Biotite — Only noticed on a few occasions as small flakes 

 enclosed in felspar and quartz phenocrysts. 



Iron ores — These occur as irregular patches and grains 

 riiostly associated with the light green chlorite patches, 

 and no doubt here represent mainly an alteration product 



