VOLCANIC PIPES 353 



Kop group. Through a dark red matrix angular or 

 sub-angular fragments of granite, quartzite, crystalline 

 limestone, and other rocks up to eight feet in length are 

 distributed promiscuously. 



At Balmoral (Katel Fontein) x in the Fraserburg 

 Division there is a circular depression in the ground 

 about 300 feet across and from ten to twenty feet deep, 

 surrounded by the truncated edges of the Beaufort beds, 

 which dip away from the breccia filling the pipe. The 

 breccia is a soft muddy blue rock containing fragments 

 of sandstone and shale, dolerite, and pieces of biotite, 

 garnet and ilmenite. The pipe is remarkably well ex- 

 posed, and the nature of the contact and the upturning 

 of the edges of the strata through which the pipe passes 

 can be very clearly seen. 



We now come to the consideration of that class of 

 pipe which is filled with the peculiar material known as 

 "blue-ground," the type that is so well represented in 

 the districts of Carnarvon, Victoria West, Britstown, 

 Philipstown, Hanover, and especially in Kimberley and 

 Barkly West. A full account of the Kimberley group 

 has been given by Gardner Williams,' 2 while short notes 

 on some of the little known pipes of Northern Cape 

 Colony will be found elsewhere ; 3 only brief references 

 therefore will be made to a few of these. 



The Kimberley Mine is the richest and deepest of the 

 De Beers group. The pipe is rudely elliptical in sec- 



1 a. c., v., p. GO. 



2 G. F. Williams, Tfo Diamond Mines of South Africa, etc., New 

 York, 1905. 



:{ G. C., xi., p. 135; xiii. 



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