222 HOLLAND: CHARNOCKITE SERIES. 



and thus comparable to the banding which often characterises rhyo- 

 lites in which patches of crystallites, or of coloured material, are 

 drawn out to thin streaks by distortion of the viscous lava along one 

 prevalent plane, the plane along which the lava flows and which is at 

 right angles to the direction of the maximum pressure ; that is, in the 

 case of the rhyolitic lava, at right angles to the direction of gravity 

 which produces the flow. 



Whether the magma became "schlierig" by molecular differ- 

 entiation before crystallization commenced, or whether it was the 

 outcome of segregation during the processes of consolidation, is 

 a matter of secondary concern as long as it is understood that the 

 distortion occurred before the rock was sufficiently solid to show crush 

 structures. 1 It is important to keep this point in mind because, whilst 

 this kind of banding is more generally due to the mere distortion 

 in one direction of a " schlierig " magma, there is another form of 

 banding of a much more perfect kind, which, in some cases at least, 

 is due to lit-par-lit injection (infra, p. 223). 



This imperfect, discontinuous, lenticular kind of banding is a 

 strong argument in itself against the idea that the banding in these 

 rocks represents relics of an original sedimentary structure, 2 whilst 



1 Apparently, from the illustrations he employs, Reyer thinks that the "schlierig" character 

 of a magma is developed whilst it is still in the molten condition : "Nur selten sind grossere 

 Massen eines Korpers wirklich homogen ; bei der Aufiosung eines Salzes treten verschieden 

 concentriite Partien der Salzlosung als Schlieren nebeneinander auf, welche sich durch 

 verschiedenen Salzgehalt, spez. Gewicht, Lichtbrechung, etc., unterscheiden. Das Meer ist 

 schlierig ; es bestehtaus verschieden concentrirten und verschieden warraen Wassermassen ; 

 die Luft ist schlierig, weil sie partienweise verschieden mit Wasserdampf, Staub, etc., 

 vermischt ist j schlecht gemischter Teig, Lava-massen, Granite sind gleichfalls schlierig; 

 kurz wohin wir blicken, die Liquida, sowie die festen Korper sind ungleich gemischt, sie waren 

 seit jeher schliarig " (Theoretische Geologie, p. 81). 



2 That the banding of the gneisses has any necessary connection with original sedimenta- 

 tion is probably held now by very few geologists ; but the idea, promulgated by the genius 

 of Lyell, naturally dies hard, and probably still influence 1 !, if unconsciously, our forms of ex- 

 pression. It is not rare to find, for instance, that the unconformity of the transition 

 schists is said to be shown by their beds being iound to overlap "the upturned edges 

 of the gneiss ". To those who regard the old gneisses as " portions of the primeval crust of 

 the globe, traces of the surface that first congealed upon the molten nucleus", or who hold 

 any such sweeping, but necessarily speculative, theory, the banding (bedding) of the gneisses 

 may convey a concrete meaning j but it is probably more profitable to work out each gneissose 

 formation for itself, and then to draw the simple inferences which are usually permitted for 

 similar phenomena presented by rocks whose characters are better understood and which 

 permit of tafe deductions. There seems no adequate reason, so far, for excluding the old 

 gneisses to the limbo of the mysterious. 



( I0 4 ) 



