412 W. M. DAVIS OBSERVATIONS IN SOUTH AFRICA 



here see some 20,000 square feet of distinctly rounded and grooved, but 

 somewhat weathered, rock surface; that at least 1,000 square feet of this 

 still preserved distinct striations of characteristic glacial quality, and that 

 the scattered remnants of Dwyka occupied a few hundred square feet. 

 The general relief of the surface was small. The direction of ice-motion 

 here indicated was to the southwest. Still better exposures of this kind 

 occur half a mile farther up the river, where the remarkable views repro- 

 duced in plate 54, figures 1 and 2, were taken and sent to me by the 

 kindness of Professor E. B. Young, of the Technical Institute, Johannes- 

 burg, whom we had the good fortune to meet at Eiverton. It may be 

 added that two of our party, Mr Lamplugh, of the British Geological 

 Survey, and Doctor Beck, of Freiberg, reported that they had seen the 

 Dwyka on a striated surface of older rock in the great open pit of the 

 Kimberley mine at Kimberley. The upper part of the pit, perhaps a 

 quarter of a mile in diameter, has sloping sides in the weaker beds of the 

 Karroo series for some 300 or 400 feet; the walls then go down vertically 

 out of sight from the surface for 1,000 feet in the harder older rocks, 

 thus indicating the form of this extraordinary "pipe" of "blue ground," 

 the matrix of the Kimberley diamonds. It is at the change from the 

 sloping to the vertical wall that the Dwyka is seen. We were told that 

 it may be examined by following the galleries that are opened there to in- 

 tercept the ground water from the overlying beds. The striated surface 

 on which it rests was reported to be diabase similar to that which we saw 

 at Eiverton. The general form of the glaciated surface was nearly level, 

 as judged from the form of the pit when seen from the surface. When 

 this is taken in connection with what we saw at Eiverton, it confirms very 

 well the conclusion reached before as to the small relief of the floor on 

 which the Dwyka rests. 



Glaciated DiuyTca floors elsewhere. — Other accounts of the striated rock 

 floor under the Dwyka have been published as follows : There are well stri- 

 ated quartzites with striations bearing south 10 degrees west near Prieska, 

 on the Orange river, in Cape Colony, first recognized by Stow (about 

 longitude 23 degrees east; see p, figure 1) and recently described by 

 Eogers (pages 155, 159, plates viii and ix) . Striations bearing south, mag- 

 netic, have been found on the Waterberg sandstones near Balmoral, east of 

 Pretoria, in the Transvaal (about longitude 28% degrees east; see b, fig- 

 ure 1 ) . These are described by Mellor, who adds that the northernmost 

 areas of the Dwyka are nearly in latitude 24% degrees (pages 117, 118) ; 

 also by Hatch and Corstorphine (page 209, figure 50). Striations bearing 

 about east and west, the direction of ice-movement not being specified, 

 occur on the Tugela river in Natal (about longitude SI 1 /} degrees east; 



