part 2] ^ or southern' eyre pexinsula. 125 



rhombic silicate is found among the dissociation-jDroducts of the 

 original pyroxene. 



The associations of biotite and hornblende around the grains of 

 iron-ore are also to be attributed to metamorphism, and perhaps 

 are developments of a declining stage of this inHuence. The iron- 

 ore has supplied a portion at least of the iron in the development of 

 biotite, and the su^^ply of potash is presumably derived from the 

 felspar, small amounts of the orthoclase molecule usually being 

 present in the constitution of the plagioclases. In the absence of 

 vcxact knowledge of the agents by which this change is brought 

 about, it is difficult to represent this biotite development by any 

 trustworthy equation. 



As to the origin and relationships of these metamorphosed 

 dolerites, it is unfortunately difficult to say much. At Kirton 

 Point, the dolerite is developed interbanded with the augen- 

 gneisses. This band runs parallel with the primary foliation of 

 the gneisses. At Boston Island, the orientation of these bands is 

 not so constant, and in some cases there is a slight departure from 

 Si parallel orientation. 



In none of these cases has a band been seen to send intrusive 

 venules into the enclosing gneiss, nor, on the other hand, are the 

 bands traversed by gneissic veins, as is so commonly the case with 

 the neighbouring amphibolites. A massive structure of the 

 dolerite might, however, account for such inaction. At first sight, 

 these rocks appear to be characteristic dyke-rocks cutting the acid 

 gneisses. When, however, the high-grade metamorphism is 

 ^established, this opinion must be considered open to doubt. 



The microscopic characters resemble in many particulars those 

 of the basalt-lavas of Skye, which, becoming entangled as inclu- 

 ^sions in the later gabbros, have been recrystallized, with develop- 

 ment of secondary granulitic augite.^ 



The production of secondary augite as a result of a high-grade 

 thermal metamorphism, has been recorded by Dr. W. F. Smeeth 

 for the Kola schists of Mysore State, where they have been 

 invaded by later granites. ^ He believes, however, that the 

 secondary augite is here developed by recrystallization of horn- 

 blende of the pre-existing hornblende-schists. A somewhat similar 

 type of metamorphism is presented by certain of the Lizard 

 hornblende-schists. The occurrence of secondary augite in members 

 of the Landewednack schists is ascribed to thermo-metamorphism 

 of these rocks by intrusive masses of serpentine.^ 



Augite is thus a typical metamoi-phic mineral of the highest 

 grades of metamorphism in rocks of this class. The possibility, 



^ A. Harker, ' Tertiary Ig-neous Rocks of Skye ' Mem. Geol, Surv. 1904, 

 pp. 53 & 116. 



2 W, F. Smeeth, ' The Occurrence of Secondary Augite in the Kola Schists ' 

 Mysore Geol. Dep. Bull. 3 (1905) pp. 1-84. 



■^ J. S. Flett, ' The Geology of the Lizard & Meneage ' Mem. Geol. Surv. 

 1912, pp. 46-50. 



l2 



