134^ GEANITE-GNEISSES OF SOUTHEEN EYEE PE]S'I]S^SULA. [vol. Ixxvii, 



pegmatites, while often associated with the amphibolites, also 

 ■occuiTed as distinct dykes cutting the Flinders gneisses. With 

 regard to myrmekite, this structure was also developed in certain 

 Palaeozoic intrusions (for instance, in the Caledonian granites of 

 Galloway). Its comparatiye rarity in later granites and abundant 

 deyelopment in Pre-Cambrian granites might be attributed — not 

 to metamorphism, but to the intrusion of the Pre-Cambi'ian 

 granites in regions of comparatively less steep temperature- 

 gradients, Avith the result that the final residual solutions could 

 act for longer periods on the already crj'stallized minerals. 



In reply to Dr. Evans, the Author stated that there was no 

 evidence in the area in question, that assimilation could account 

 for any of the features shown by the pyroxene-granites. He 

 regarded the developinent of pyi'oxene as a resultant of a com- 

 parative poorness of magmatic water, whereby the metasilicate 

 molecule did not suffer degradation to the orthosilicate type of the 

 micas. He had not found any perthitic intergrowths in these 

 rocks, in which the plagioclase was as calcic as andesine. 



