GEOLOGY OF THE JENOLAN CAVES DISTRICT. 361 



Although the analysis only gives traces of pyrites the 

 sections certainly show more, this is due to the fact that 

 the material chosen for analysis was carefully selected, 

 any showing pyrites being rejected. 



The calculation of the norm also indicates that the bases 

 present are not sufficient to satisfy all the alumina, the 

 balance 3*47 per cent, entering into the norm as corundum. 



The microscopic examination shows the rock to be much 

 altered, particularly the felspars, which are now repre- 

 sented largely by a sericite-like material with some kaolin. 

 It illustrates an example of metasomatism, through the 

 action of percolating superficial water acting on the sili- 

 cate of lime in the felspar, resulting in a loss of lime and 

 a gain in alkalis, particularly potash, and probably alu- 

 mina, as indicated by the development of sericite and kaolin. 



The result of Termiers 1 investigations has shown that in 

 general the above action has resulted invariably in the loss 

 of lime and a gain in alkalis. Silica remains approximately 

 constant, the alumina varies and the magnesia fluctuates 

 between a small loss or gain. 



Allotting all the hypersthene molecules together with 

 some of the anorthite to form the augite and hornblende 

 present in the rock, the average plagioclase would be 

 about andesine of a basic character. Plagioclase would 

 also largely predominate over orthoclase. In arriving at 

 the actual mineral constitution from the norm it is also 

 necessary to bear in mind that this rock, particularly the 

 felspar, has undergone a good deal of alteration, which has 

 probably resulted in a loss of lime and a gain in potash and 

 alumina, giving to the norm a higher percentage of ortho- 

 clase than actually existed in the unaltered rock. 



1 Sur T elimination de la chaux par metasomatose dans les roches 

 eruptives basiques de la region du Pelvoux, P. Termier. Bulletin de la 

 societe Geologique de France, series 3, Vol. xxvi, 1898, p. 165. 



