part 2] OF souTHERisr eybe peninsula. 89 



in length. A green diopsidic pyroxene of high extinction-angle 

 may sometimes form an important constituent of these hornblende- 

 pegmatites. 



The aplites are fine pink-weathering rocks, consisting of 

 quartz and microcline or an acid plagioclase. A prominent aplite 

 near the Kirton-Point jetty is an interesting type in which the 

 dominant felspar is albite, although microcline is also present. 

 The rock has been veined by epidote-strings, and the albite is 

 filled with little crystals of an epidote of the clinozoisite type. 

 These grains show a noteworth}^ variation of interference-colour 

 within the same crystal, and the presence of parallel growths of 

 epidote and clinozoisite is indicated. The albite shows no deve- 

 lopment of the chequer-structure (schachbrett-albit), which 

 is a common feature of the albitites and soda-aplites of South 

 Australia. 1 



[b) Gneisses of the Sleaford Area (Sleaford Gneisses). 



The principal distinguishing feature of the Sleaford gneisses is 

 the presence of garnet, which is widely developed throughout the 

 accessible exposures of these rocks in the hundred of Sleaford. 

 The prevailing rock-type is a garnet-biotite-gneiss, consisting of 

 quartz, microcline, plagioclase of the composition of oligoclase- 

 andesine or andesine, biotite, garnet, and the accessories apatite, 

 magnetite, and zircon. Myrmekite is as frequent a constituent of 

 these rocks as of the Lincoln Series, and shows the same relations 

 to the adjacent minerals. 



Types containing a member of the pyroxene group of minerals 

 are also more frequently met with than in the Port Lincoln 

 district. We thus find garnet-h3rpersthene-gneisses, as well as 

 diopside varieties. Hornblende is also a common mineral, but 

 is not so abundantly developed in the granite-gneisses as in the 

 area already described. 



The garnet is a pink almandine, occasionally reaching half an 

 inch in diameter, but commonly much less. Under the micro- 

 scope it is colourless to pink, and invariably isotropic, as is usually 

 the case with pyrogenetic garnets. 



In a number of the pyroxene-bearing types of gneisses, the 

 garnet ma}" form a corona round other minerals, being developed 

 in this manner around such minerals as magnetite, hornblende, or 

 pyroxene. In some of these examples there is sufficient evidence, 

 if such were necessary, that this is a primary corona structure, 

 its relation to the associated minerals enforcing this conclusion on 

 chemical grounds. 



In addition to the minerals already mentioned, sphene and a 

 mineral agreeing most closely with allanite are occasionally found. 



The most abundant type of pegmatite in this region is a 



1 C. E. Tilley, Trans. Eoy. Soc. S. Austr. vol. xliii (1919) pp. 316-41 ; 

 also W. R. Browne, ibid. vol. xliv (1920) pp. 1-57. 



