part 12 J OF SOUTHEKN EYRE PEXI^sSULA. 87 



former mineral. These inclusions present some of the features 

 of the haematite lamella? in the aventurine-felspars described by 

 0. Andersen. 1- 



A distinct type of inclusion is afforded by the presence of a more 

 or less regularly oriented system of orthoclase-lenticles in some 

 of the plagioclase crystals, corresponding to the 'antiperthite' of 

 F. E. Suess.2 These are usually developed parallel to the vertical 

 axis, and would appear to correspond to the spindle-shaped grains 

 of albite in microcline ; they are assignable to an unmixing from a 

 homogeneous solid solution consequent on cooling. 



The peculiar quartz-plagioclase intergrowths which have been 

 variously designated as ' quartz vermicule ' b}'' F. Fouque & A. 

 Michel-Levy,^ and later by Dr. J. J. Sederholm'^ as ' myrmekite,' 

 are a constant feature of the Flinders Series as a whole. The pro- 

 perties and nature of this myrmekite correspond very closely to the 

 detailed description given by Dr. F. Becke.^ In particular we may 

 consider its distribution with regard to the microcline-felspar. The 

 intergrowth is developed at the junction of the plagioclase with the 

 potash-felspar, and without exception presents a convex surface to 

 the microchne mineral, the wormlike quartz being usually roughly 

 arranged perpendicularly to this surface. This convex and invasive 

 character of the myrmekite into the m.icrocline is strong evidence 

 that the potash mineral has been replaced. The plagioclase of the 

 in3a-mekite ground is in optical continuity with the plagioclase core. 

 Becke has quantitatively measured the ratio of quartz to plagio- 

 clase, and finds that this index increases with the anorthite content 

 of the felspar ; in accordance with this evidence, he suggests that 

 the changes involved ma}^ be represented by the equations 



KAlSisO. + Na = NaAlSi,,0, -f- K, 



2KAlSi30, + Ca = CaAl,Si,0,-f 2K + 4Si02. 



In treating of the origin and time of development of myrmekite 

 Dr. F. Becke, ^ in a later paper, says : 



' Gesteine mit ganz reinen Erstarriuigsstrukturen sclieint er zii meiden .... 

 Ich kann ang'eben, dass icli ihnin Schliffen von Granit von Predazzo vergeblieh 

 gesucht babe. Auch in Schliffen des Granits vom Brocken habe ich. ihn nicht 

 finden konnen. Die Myrmekitbildnng ist wohl anf Tiefengesteine undkrystal- 

 liniscbe Schiefer beschrankt. . . . Wenn man aus diesen Tatsachen einen Scbluss 

 Ziehen diirfte, so ist es der, dass die Myrmekitbildnng" sich in einer Phase der 

 Gesteinsbildiing zn vollziehen scheint, die sich nnmittelbar an die Erstarrting 

 anschliesst, also zu einer Zeit, wenn die Temperatnr noch der Erstarrungs- 

 temperatur nahe steht iind noch Losungsmittel im Gestein vorhanden sind.' 



The comparative rarity of this intergrowth in the later Pre- 

 Cambrian and Palijeozoic granites, when its common occurrence in 



^ Amer. Journ. Sci. ser. 4, vol. xl (1915) pp. 351-98. 



- Jahrb. K. K. Geol. Eeichsanst. vol. liv (1904) p. 417. 



'■^ ' Mineralogie Micrographique ' 1879, p. 193. 



4 Bull. Comm. Geol. Finl. No. 6, 1899. 



^ Min. Petr. Mitth. vol. xxvii (1908) pp. 377-90. 



^ Denkschr. K. Akad. Wissensch. Wien, vol. Ixxv (1913) pp. 134-40. 



