part 2] OF SOUTHERK^ EYRE PENINSULA. 91 



best exposures of these banded gneisses are seen at the headland 

 north of Fishery Bay, and again on the coast near the Red Banks, 

 Western Sleaford. At the latter localit}^, the gneiss shows bands 

 of differing composition: dark bands containing larger percentages 

 of ferromagnesian minerals are seen to alternate with white aplitic 

 or quartzose bands, the whole being closely contorted. These are 

 crossed, perpendicularly to the banding, by reddish pegmatite- 

 veins. In some cases the white aplitic bands are drawn out into 

 lenticles, and at the points of attenuation the red pegmatite -veins 

 have been intruded transversely (see fig. 5, p. 90). 



Xenoliths of Sedimentary Origin in the 

 Sleaford G-neisses. 



Fragments of the Hutchison Series engulfed in the Flinders 

 Series of gneisses are to be found sparing]}^ developed at various 

 points in the Sleaford area. The most notable locality is the 

 headland north of Fishery Bay. From this locality, diopside- 

 rocks have already been described in an earlier paper. There are, 

 however, certain additional xenoliths closely associated with these 

 which have not as yet been described. These xenoliths are repre- 

 sented by knots and bands in the gneisses, the knots reaching 

 a diameter of as much as 5 inches. They are very rich in garnet, 

 and on account of their more resistant weathering project from an 

 otherwise fairly smooth face. 



As seen under the microscope they consist of pink garnet,, 

 green spinel, biotite, magnetite, and pyrites. Garnet is the 

 most abundant constituent, but is intimately intergrown with 

 the spinel. This mineral is dark green, forming rounded inter- 

 growths, and often enclosing magnetite-g'ranules. The line of 

 demarcation between the magnetite and the spinel is sharp, and 

 without any gradation of colour to suggest the presence of any 

 extended mutual solubility of the group FeO.Fe^Og . . . FeO. Al^Og. 

 The spinel is a member of the pleonaste-hercynite group (see 

 fig. 6, p. 92). 



The biotite occurs in well-developed flakes, showing a pale-yellow 

 to reddish-brown dichroism. There are no pleochroic haloes in the 

 biotite where the spinel is enclosed in this mineral. There are, 

 however, sometimes present minute pleochroic haloes around zircon 

 enclosures. Some intergrown plagioclase of andesinic composition 

 is present. 



In the gneiss associated with these knots are sometimes found 

 individual crystals of garnet, containing intergrowths of green 

 spinel, and in addition sillimanite may be similarly present (see 

 fig. 7, p. 92). Tiiese are fragments of an original xenolith, which 

 has become disintegrated by the fluid magma. 



There are narrow schistose bands in this locality, consisting of 

 biotite and garnet with subordinate potash-felspar. The garnet 

 in these bands shows an anomalous anisotropism. 



The presence of such highly-aluminous minerals as spinel and 

 Q.J.G.S. No. 306. I 



