Vol. 70.] OX THE VEEDEFOET GEAXTTE. 335 



Discussion. 



Dr. F. H. Hatch said that he had the more pleasure in con- 

 gratulating the Author on a useful piece of geological work, since 

 he had had the advantage of his assistance when investigating the 

 mineral resources of Zululand for the Xatal Government. He was 

 also not unacquainted with the ground which formed the subject of 

 this paper, and he could from his own knowledge testify that the 

 difficulty of laying down geological observations, on maps on which 

 only farm-boundaries and no topographical details were given, had 

 been admirably surmounted. Moreover, the results obtained were 

 of the greatest importance to Transvaal geology generally. Hitherto 

 the Yredefort Granite had been considered as one of a series of 

 ancient intrusions in beds of Swaziland age, and older than the 

 AYitwatersrand Series. The Author showed that not only was the 

 Yredefort Granite intrusive into the AYitwatersrand Beds, but also 

 into diabases which probably were genetically connected with the 

 Yentersdorp Yolcanic Series. This would make the granite post- 

 Yentersdorp, and possibly of the same age as the Red Granite of 

 the Boschveldt, which was later than the Pretoria Beds. Since in 

 the Red Granite there had been numerous discoveries of stanniferous 

 deposits, it might prove worth while also to search the Yredefort 

 Granite for tin-ore. 



Dr. J. \Y. Evaxs thought that the paper showed the value of 

 detailed mapping in deciphering the true structure of a district. 

 Once more a supposed 'fundamental" granite, upon which ancient 

 sediments were believed to rest unconformably, had proved to be 

 intrusive in them. The evidence of overhead sloping was of great 

 interest. It was easy to understand how fragments broken from 

 the roof covering a granite magma would sink through the latter, 

 which would have a very low density — not only on account of its 

 acid character, but also because of the large amount of water and 

 other volatile constituents that it appeared invariably to contain. 

 A basic magma, on the other hand, -would have too high a specific 

 gravity to permit as a rule of overhead sloping of this character. 



Mr. F. P. Mexxell said that, although he had no special know- 

 ledge of the particular locality referred to in the paper, it was 

 evidently of great interest. There was a possibility besides that 

 which the Author had pointed out of connexion with the Transvaal 

 ' Red Granite." The granite-masses of the Cape Province were 

 intrusive in the Malmesbury Slates ; and although these were, at 

 one time, supposed to be the bottom of everything, they were now 

 considered as probably equivalent to the Pretoria Beds, so that they 

 really came quite high up, as things went in South Africa. The 

 southern granites might, therefore, be considerably younger than 

 the northern granites, and the Yredefort mass might fall into the 

 former group. The Author did not seem to have given any petro- 

 graphical account of the rock, and it would be interesting to know 

 whether it was characterized by microcline like the great northern 

 granite-masses, or orthoclase like those of the Cape. 



Q. J. G. S. No. 279. 2 a 



