80 ME. C. E. TILLEY ON THE GEAlNTTE-aNEISSES [vol. Ixxvii^ 



For descriptive purposes the basic bands of the Fhnders gneiss 

 can be subdivided as follow^s : — 



(1) Amphibolites and hornblende- schists. 



(2) Pyroxene -bearing types (pyroxene-granulites). 



(3) Bands of metamorphosed sediments. 



(4) Metadolerites. 



Dealing now with the broader features of the Flinders Series,. 

 I may point out that in the southern area the foliation of the 

 granite-gneisses is predominantly north and south, with a vari- 

 able swing to a little east or west of this prevailing direction. 

 The dip or inclination of the foliation is at a high angle, in some 

 cases practically vertical, or at a high angle to the west. In the 

 northern area, the foliation of the gneisses has swung to the north- 

 east, but the steeply-dipping foliation-planes are still developed. 



The abundant development of this gneissic foliation shows that 

 crystallization accompanied movement of the magma. These 

 movements in the magma were not limited to the period in which 

 the magma was largely liquid, but continued during crystallization, 

 with consequent orientation of elongate crystals such as porphyritic 

 felspars, biotite, and hornblende, and was progressing when crystal- 

 lization had well advanced. 



There are evidences of protoclastic structures to be met Avith in 

 the different areas. Such movements in the later stages, Avith a 

 growing rigidity of the mass, may be developed in directions 

 oblique to the main direction of foliation, and along these planes of 

 shear the gneissic foliation is locally deflected, the plane of shear 

 being represented by a finer granulitic type. Such features indicate 

 local movement when the gneiss had reached a stage of considerable 

 viscosit}^ 



The gneissic foliation is not a constant feature of the granites. 

 An imperceptible gradation from perfectly massive tjqoes through 

 varieties in which a foliated structure is just discernible, to strongly 

 developed augen-gneisses or banded types can be traced. The 

 separation of different types is, therefore, precluded, and the 

 gneissic foliation which is set up is not wholly assignable to any 

 one cause, as will be indicated hereunder. The massive and foliated 

 types are different phases of the one rock-miass. In some cases the 

 dominant gneissic structure can be ascribed to hybridization, or to 

 the disruption of a pre-existing dark band through intimate pene- 

 tration by the later acid intrusion. It is quite clear that the 

 gneissic structure cannot be regarded in any way as a metamorphic 

 feature, and the foliation is essentially that described for primary 

 gneisses of other regions. 



The granite-gneisses have been intersected by a later though 

 comagmatic series of pegmatite- and aplite-dykes, which often 

 show no trace of gneissic foliation, and cut the gneisses at right 

 angles to the prevailing direction of foliation, the east-and-west 

 joint-planes being often utilized as fissures. 



