38 



rocks, all very similar in general characters and all presumably 

 coeval. In two localities the presence of associated acid in- 

 trusives has been proved. In his paper on the Mount Re- 

 markable area (30) Prof. Howchin mentions aplites and 

 porphyritic acid dyke rocks in close relationship with the basic 

 intrusives. In an appendix to the paper Thiele describes the 

 porphyritic rock as a quartz keratophyre and the aplite as a 

 microcline-bearing type. The former is very similar chemically 

 to the Port Elliot soda aplite, while the latter, though not 

 analysed or described in detail, is clearly potassic. 



Through the courtesy of Prof. Howchin the present 

 writer was able to examine microscopically a dyke rock from 

 Blinman, evidently co-magmatic with the diabases and 

 granophyric diabases described by Benson, (^i) The rock is 

 fine grained and pink coloured, non-porphyritic and much 

 carbonated, but composed largely of albite, with a consider- 

 able proportion of interstitial quartz. It would be placed 

 with the quartz keratophyres. 



These instances seem to the writer to lend some weight to 

 the supposition of a genetic relation between the dolerites and 

 certain of the aplites of Encounter Ba}^. 



(2) MINERALOGICAL VARIATIONS IN THE PRINCIPAL ROCK TYPES. 



The following table indicates at a glance the variation of 

 the principal constituents : — 





Quartz 



Porphy- 



Even- 



Albite 





mica 



ritic 



grained 



mica 





diorite. 



granite. 



granite. 



syenite. 



Quartz 



.. 34 



29 



37 



2 



Microcline ' . 



.. 



23 



47 







Plagioclase 



.. 44 



30 



9 



85 



Biotite 



. 21 



16 



5 



9 (chlorite) 



Sp. gr. of rock 



2-716 



2-673 



2-615 



2-635 



Considering the first three rocks, we find some notable 

 features. There is very little variation in the quartz content, 

 in spite of the range of Si02 percentage. Plagioclase and 

 biotite, the minerals of highest specific gravity, decrease in 

 amount towards the acid end of the series, while microcline, 

 the lightest mineral, shows a very marked rise, plagicclase the 

 while becoming more acid. All this is another way of express- 

 ing the fact that the order of commencement of crystallization 

 for this particular magma has been one of decreasing specific 

 gravity, and this is reflected in the decreasing specific gravity 

 of the rocks. 



(30) Trans. Roy. Soc. S. Austr., vol. xl., 1916, p. 545. 



(31) Trans. Roy. Soc. S. Austr., vol xxxiii., 1909, p. 226. 



