82 Transactions of the South African Philosophical Society. 



plosions took place that drilled holes only a few hundred feet in 

 diameter through many thousands of feet of rock ; these holes were 

 finally filled up either with serpentinous breccia, a mixture of such 

 matter and ultra-basic igneous rock, agglomerates and tuffs of little 

 else than comminuted sedimentary beds, or with igneous rock alone 

 as at Spiegel Eiver and some of the Sutherland Commonage vents. 



Whether the present surface section of a pipe is surrounded by 

 Pre-Cape beds as at Pretoria and some of the West Griqualand 

 occurrences, or by different stages of the Karroo formation as at 

 Kimberley and Sutherland, depends entirely on the relative progress 

 of denudation. In no case has the original crater been preserved. 

 We are, indeed, entirely ignorant as to whether piles of agglomerates 

 and tuffs were thrown out at the surface. Had lava streams of any 

 great thickness issued from these volcanoes they would probably 

 have left outliers to bear witness of their former extent, for the 

 Stormberg lavas have a wide distribution, and they are older than 

 the western volcanoes. It is of course quite possible that evidence 

 bearing on these questions is still preserved in South Africa, but 

 remains to be discovered. 



Pipes of the nature of those described in this paper are not 

 abundant in other parts of the world. At Bingara * in New South 

 Wales there is a pipe of which the contents bear a striking general 

 resemblance to those of some of the South African pipes. In a dull 

 greenish blue matrix there lie fragments of claystone, melaphyre, 

 augite-picrite, eclogite, and many minerals including pyroxenes, 

 garnet, felspar, and pleonaste. The age of this vent is probably 

 Tertiary.f 



In North America, dyke rocks strikingly similar to the blue-ground 

 of Kimberley have been met with at Syracuse, New York, and in 

 Kentucky. They have a distinct fragmental structure, though 

 formed of minerals that generally occur in ultra-basic igneous rocks, 

 and although they occur in dykes.]: 



In a paper dealing with the occurrence of eclogite boulders in the 

 Newlands Mine Professor Bonney § takes the view that the rounded 

 and apparently waterworn boulders of eclogite were obtained from a 

 conglomerate — the Dwyka conglomerate — during the explosion which 

 caused the pipe. This view cannot be maintained, at any rate the 



* See G. W. Card, " An Eclogite-bearing Breccia from the Bingare Diamond 

 Field," Eec. Geol. Surv. of New South Wales, vol. vii. pt. ii. 1902, pp. 29-39. 



f " The Mineral Resources of New South Wales," E. F. Pittman, 1901, p. 393. 



\ The original descriptions of these rocks are not available to us, but a good 

 account by Professor Bonney is printed in " Genesis and Matrix of the Diamond,' 

 p. 58. 



§ Geol. Mag., 1899, p. 321 



