96 ME. C. E. TILLEY 01^ THE GEAXITE- GNEISSES [vol. IxXVU, 



These essentials being granted, it is possible to account reason- 

 ably for the features presented by the gneisses of the Flinders Series. 

 Let us take first the case of a heterogeneous magma arising from 

 differentiation, and we shall note that under this head come those 

 gneisses which show evidences of a IIoav- structure as distinct from 

 a gneissic banding. The elongate minerals, such as biotite and 

 hornblende, are the principal determining features of this foliation. 

 Flow -movements arising during the crystallization of these 

 minerals, when the quartz and felspars were largely in solution in 

 the magma, are responsible for the orientation and alignment of 

 these minerals. 



In considering the explanation of the origin of banded gneisses 

 an additional feature is to be remarked. Examined "under the 

 microscope, the banded gneisses show an alternation of dark bands 

 richer in the earlier-formed minerals, with bands principally 

 composed of quartz, or quartz and felspar. 



In all cases it is evident that quartzose, or quartzo-felspathic 

 bands have crystallized later than the darker bands, the relations 

 in some cases beino- visibly intrusive. It seems clear that we have 

 here a magma in which crj-'stallization has progressed far. The 

 magma may be pictured i-ather as a crystalline mesh work, in which 

 the interstices are filled Avith the quartzose or quartzo-felspathic 

 solutions. It is not difficult to conceive of a meshwork of this, 

 description in which the crystalline mass is penetrated by inter- 

 stices carrying the residual lluid in continuous contact, much as in 

 a sponge, and a mass very susceptible to shearing. A consideration 

 of the influence of shearing movement on such a crystalline mesh- 

 work indicates that a prime factor is the mobility of the interstitial 

 liquid. Under deformative movement, cavities may be produced, 

 which on account of that same mobility are immediately filled with 

 liquid from the neighbouring interstices of the mesh. Such, 

 cavities would possess in general a lenticular shape, corresponding 

 to the direction of the impressed force. Continued action of this 

 kind, under a constant shear-direction, and moderate temperature- 

 gradient, Avould finally result in the production of a banded structure 

 of definite orientation, and of such a mineral composition of 

 alternating bands (which is in accordance with the progressive 

 crystallization, the bands bearing very similar mutual relations) 

 as that Avhich obtained between the orio-inal crystallized minerals 

 and the liquid at the initiation of this process. 



It is to be remarked that not only is a parallel alignment of the 

 banding thus established, but that the elongate minerals of the 

 bands adjust themseh^es to this orientation, and thus we may see 

 where individual crystals of the mesh become involved in the liquid 

 residuum. There is, consequently, foliation superposed on a more 

 or less unidirectional banding. The processes at work in this 

 shearing are clearly unfavourable to any gravitative separation of 

 crystals. 



It may further be suggested that if, when an approximately 

 parallel banding is established, and before consolidation has 



