part 2] OF SOUTHEKIS" EYRE PENINSULA. 85 



may appear massive, and the foliated structure is only brought out 

 when a considerable area of the rock is viewed in the field. 



There are certain types which characterize different areas : for 

 example, garnet enters as an important constituent into the gneisses 

 of the hundred of Sleaford, Avest of the Hutchison Series, and has 

 not been observed in the gneisses of the Lincoln area ; and there 

 are other features which justify the division into two groups for 

 descriptive purposes. But the appearance of garnet in the Sleaford 

 rocks cannot be considered as implying any important distinction 

 with regard to the origin of the rock-types. 



J. shall, therefore, describe the Flinders gneisses under 



(a) The gneisses of tlie Lincoln area, to include the gneisses found east 



of the Hutchison Series ; and 



(b) The gneisses of the Sleaford area, to include types developed in the 



hundred of Sleaford as far west as the Red Banks on the western 

 shore of this hundred. 



{a) Gneisses of tlie Lincoln Area (Lincoln Gneisses). 



The most characteristic rock of the Lincoln area is a coarse- 

 grained augen-gneiss, which is developed in the type-section of 

 Kirton Point, near the jetty of Port Lincoln. 



The typical rock shows augen of felspar, or of felspar and 

 quartz, usually aggregates and not single crystals, measuring up 

 to an inch in len^'th. The dominant ferromao'nesian mineral seen 

 in hand-specimens is a flaky biotite, but hornblende may also be 

 discerned. The foliation is in a north-and-south direction, with a 

 vertical dip. 



Under the microscope the rock is seen to consist of quartz, 

 plagioclase, microcline, biotite, hornblende, apatite, sphene, and 

 magnetite. The quartz shows undulose extinction, and is some- 

 times singularly free froui solid inclusions. There are the charac- 

 teristic developments of pores along cracks of secondary origin. 

 The plagioclase is developed with both albite and pericline twin- 

 lamellation, and the properties are those of an andesine. Zonary 

 banding is rarely seen. The andesine is characterized by rodlike 

 inclusions oriented along definite cr3'stallographic directions. Some 

 of these have a platy habit, and are brownish, and of high refractive 

 index, corresponding to rutile. In some examples these inclusions 

 may also be present, but less characteristically in the quartz- grains. 

 The potash-felspar shows considerable variation in the different 

 tj'-pes, but is usually a microcline often exhibiting a microperthitic 

 intergrowth of albite. Mjn-mekitic intergrowths of quartz and 

 plagioclase are often well developed, and typicall}'' in association 

 with embayed grains of orthoclase. 



Biotite is usually the most abundant ferromagnesian mineral. 

 The pleochroic scheme for this mica is 



X = straw-yellow ; Y = Z = dark brown. 



The included minerals are idiomorphic prisms of apatite, a few 

 grains of sphene, and minute zircons with pleochroic halos. 



