11(5 MB. C. Z. TIILET OX THE GEAXTTZ-&yi:iSS£S vol. IxXTU^ 



been observed amphibolites which can be safeir interpreted by their 

 field-occurrence as posterior drkes.^ 



Of these dominantlv pyroxene-bearing basic band^, those met 

 Avith iu the Sleaford area are true inclusions, as seen from their 

 relations to the surrounding srneisses. These have been petro- 

 Erraphieally described under the heading of pyroxene-granuhtes. 

 There ai-e, however, in addition to these, certain basic Ijands of 

 dyke-origin comagmatic with the gneisses, and of basic chamoekite 

 composition. Such have been observed cutting the pvroxene- 

 bearing granites of the hundred of Flinders near West Point. 



TTith regard to the earlier pyroxene-granulites, there are no 

 criteria of absolute certainty that can be utilized to determine 

 the character of crystallization, whether igneous or metamorphic. 

 Their srranulitie texture, the frequent absence of twinning in the 

 plagioclases, and the presence of garnet, are certainly not valid 

 indicators of a metamorphic recrystallization. Identical features 

 can l>e recognized in the well-known igneous chamoekite series of 

 India. 



ATII. Thz AIeiai clzsites or tsz Lestcolx Akea. 



(a) General Description. 



Dark bands of doleritic aspect are intercalated in the acid 

 gneisses at Kirton Point; and on the western shores of Boston 

 Island, as at other localities, there are rocks which resemble these^ 

 and are representatives of this group. 



Dealing now with those the doleritic origin of which is un- 

 doubted. I may first notice those of Kirton Point. Sir Douglas 

 Mawson obviously refers to one of these rocks in his short note on 

 the geological features of Eyre Peninsula,- where he records 



'the ocemrence of a biotite-beariiig' pflotasitic dolerite-djke about 30 feet 

 wile rniming -with the series .... it cannot be Tery andent, certiiiil- r :t 

 conLpaiable in this respect with the intmded rocks.' 



A study of thin sections of this and related bands, however, 

 reveals that these rocks have undergone a high grade of meta- 

 morphism, a conclusion wholly unsuspected from a mere examin- 

 ation of hand-specimens, and one which is likely to shed quite 

 another light on the rocks, as regards both their origin and their 

 antiqidty, in comparison with the statement quoted ab^ve. The 

 microscopic study of this series is one of extreme interest, and a 

 detailed account of their petrographic features is therefore 

 warranted. 



- Petrographic treatment of these posterior dyke-rock? is irL.^T .. 'zr.rD. a 

 further study has been made of their occurrence in the field. 



- Trans. Eoy. Soc. S. Austr. toL xxyi (1907) p. 73. 



