part 2] OF SOUTHEEIs' EYRE PENINSULA. 95 



water in jDarticular, in comparison with biotite-granites : the 

 function of M^ater in the case in question involving a degradation 

 of poi}^- and metasihcate molecules, for a high content of water 

 favours not only the orthoclase degradation, but that of the 

 metasilicates of lime and the fervomagnesian elements. The 

 presence of hjq^ersthene, diopside (the latter both in the granites 

 and pegmatites), and perhaps even hornblende itself (but to a 

 much less degree), is strongly suggestive of a relative water 

 deficiency in the crystallizing magma. Thus the equation 



(3) K2(FeMg)^Al,Si,Oi2 + 5SiO,;^2(Mg-Fe)Si03 + 2KAlSi30^ 



might be represented as a reversible equilibrium in the magma, the 

 reaction proceeding to the left in the presence of abundant water, 

 and, with a deficiency, to the right, resulting (in the latter case) in 

 a saturation of the magma with the metasilicate molecule, and its 

 consequent crystallization : or the equation representing the asso- 

 ciation of h^^Dersthene and garnet, a not uncommon occurrence in 

 the Sleaford gneisses, as of the tj'-pe 



<4) 2H3K3Mg2re,Al6(SiO J, + 12SiO,^6KAlSi30, + 3Fe,MgAl2(SiO,)3 

 ■+ re,Mg(Si03)3 + 3H20. 



The conditions favouring the right-hand side of the equation are, 

 as before, high pressure and a relative deficienc}^ of volatile water. 



The gneissic foliation and banding of these gneisses of the 

 Flinders Series presents a problem which has arisen in the in- 

 vestigation of most Archaean areas of the world, and it has long 

 been recognized that gneissic foliation may arise in various ways. 

 When we deal w^th the structui-es revealed in the Flinders gneisses 

 of Southern E3a'e Peninsula, the conclusion is enforced that the 

 features exposed in this tract can only be ascribed to conditions 

 obtaining during the primary consolidation of these rocks, in 

 contrast with any foliated structure arising through metamorphic 

 influences, subsequent to the consolidation of the rocks themselves. 

 Even so, however, there are various ways in which a gneissic 

 foliation may be impressed during the crystallization period, and 

 representatives of these different modes of origin are to be recog- 

 nized W'ithin the group in question. 



It is obvious that the first requisite for the setting-up of a 

 o-neissic foliation or bandino- in a fluid mao-ma is a mas^matic 

 heterogeneity. Heterogeneity in a magma can arise : 



(a) by differentiation in which a fluid magma contains solid crystals of 

 earlier-formed minerals, which have not settled by gravitative processes ; or 



(b) by the presence of foreign material within the magma. 



Under (b) the only case that calls for consideration in the 

 Flinders Series is the presence of inclusions of earlier-formed rocks, 

 as the widespread amphibolites and hornblende-schists. 



The second requisite is deformation, or a shearing movement 

 giving rise to flow of the magma. 



