part 2] OF SOUTHEEIN" EYKE PENINSULA. 9/" 



advanced too far, the direction of shear is for any reason locally 

 deflected, contortion and buckling of the bands themselves — and 

 more particularly of those later quartzose and quartzo-felspathic 

 bands — would ensue. With a more pronounced deflexion of the 

 shear-direction at the latest stages of consolidation, the flnal 

 residuum of liquid might be exj^ected to crystallize in narrow and 

 interrupted lenticles aligned with the new direction of shear. 



There remains for discussion the third type, in which hetero- 

 geneity has arisen by the presence of foreign enclosures. The best 

 examples of gneissic banding arising in this way are the classic 

 Tertiary gneisses of the island of Kum, as described by Dr. A. 

 Harker.i 



In the gneisses of the Flinders Series, the development of 

 gneissic foliation or banding from the presence of enclosures is not 

 on any extensive scale, but it may become locally important. The 

 foreign enclosures in this case are pre-existing amphibohtes or 

 hornblende-schists. In the first place, it will be noted that those 

 amphibolite-bands Avhich assume an elongate habit are (with 

 few exceptions) oriented with their longer axes parallel to the 

 flow-movements in the liquid magma. It will be further observed 

 that many of these inclusions are themselves foliated, and in 

 general parallel to their longer axes. During incorporation by 

 the fluid magma, they have been injected along theif foliation- 

 planes by the more fluid portions, and this injection has occasion- 

 ally been so intimate as to convert the foliated basic rock into 

 an injection hornblende-gneiss. Where such masses of amphi- 

 bolite have been torn off or separated from the main body of the 

 amphibolite, the resulting hornblende-gneiss of hybrid origin may 

 be indistinguishable from the main body of the normal gneiss ,- 

 but the process can be traced in its entirety around large masses 

 of engulfed amphibolites.^ 



With regard to features of metamorphic origin in the Flinders 

 gneisses, these cannot be regarded as developed on any great 

 scale. The metamorphism following the consolidation of the 

 gneisses is of a comparatively low-grade mechanical type. This 

 finds expression in the development of marked strain-shadows in 

 the quartzes and felspars, the undulose extinction in extreme cases 

 being accompanied by a mechanical mortar- structure. Even these 

 features appear to be localized rather than uniformly spread, and 

 are recognized from particular areas or belts which have succumbed 

 to the effects of stresses. 



An interesting feature is occasionally developed where mortar- 

 structure is dominant, in the presence of a fine myrmekite, which 



1 Q. J. G. S. vol. lix (1903) pp. 207-15. 



- The subject of primary gneissic banding- has been discussed by N. L. 

 Bowen and F. F. Grout, with special reference to the Duluth Lopolith, in 

 recent volumes of the Journal of Geology: Bowen, vol. xxvii (1919) pp. 411- 

 26 ; Grout, vol. xxviii (1920) pp. 255-64 ; Bowen, ihid. pp. 265-66. 



