98 THE PRE-CAMBRIAX ROCKS OP MOZAMBIQUE. [vol. lxxiv. 



Mr. J. F. N. Green said that the paper was of great value to 

 those interested in the Archaean plateau of Africa. He especially 

 welcomed the application of a new method of verifying the Archaean 

 succession. While there was some resemblance to the Haliburton 

 (Canada) rocks, as also to the similar occurrences in India and 

 Finland, the likeness was not well marked, since he understood that 

 oligoclase was not predominant in the biotite-gneisses, which, 

 unlike the Haliburton oligoclise-gneisses intruded into amphibolite 

 and limestone, were unaccompanied by fringes of nepheline- and 

 other alkali-syenites. On the, other hand, in West Africa all these 

 phenomena were reproduced, and it seemed that the Archaean of 

 Africa, as of other continents, presented two distinct types of main 

 granitic intrusion. He noted the statement that altered arenaceous 

 sediments only occurred on the coast ; and remarked that altered 

 argillaceous sediments seemed to occur chiefly on the outer edges of 

 the plateau, as on the Gold Coast and near Gondokoro, from which 

 latter locality a bit of garnet-kyanite-schist had been given to 

 him. The paper was an important contribution, not only to 

 African, but also to general, geology. 



The Author thanked the Fellows for their kind reception of 

 his paper, and expressed his gratitude for the complimentary 

 remarks made. 



In reply to Dr. Evans he said that he had been careful not to 

 commit himself to the hypothesis put forward in explanation of the 

 origin of the gneisses and their foliation. In the paper itself an 

 alternative Irypothesis — not involving the injection of granite- 

 magma into pre-existing schists — was outlined, but neither view 

 was suggested as being more than a possibility worthy of discussion. 

 Even if the biotite-gneisses were composite, there could be no doubt 

 that they were predominantly of granitic origin. It was necessary, 

 however, to recognize frankly that the problems involved were still 

 far from being completely solved. 



Answering Mr. Green, the Author said that, except in the horn- 

 blendic rocks, oligoclase was subordinate to orthoclase or microcline 

 in the gneisses. It was of interest to notice that in the banded 

 varieties oligoclase was an abundant mineral in the melanocratic 

 (biotite-rich) bands and streaks, whereas it was no more than an 

 accessory in the leucocratic or felsic portions of the rocks. 



[May 6th. 1919.] 



