VOLCANIC PIPES 355 



which are connected underground by a narrow dyke of 

 hard blue-ground. Some of the boulders of eclogite 

 from these mines were found to contain small diamonds. 1 

 This matter will be referred to again later. 



In outline some of the pipes are more or less oval, 

 but frequently they are much elongated or irregular in 

 shape, or may even form long dyke-like masses cutting 

 through the country rock. They can generally be de- 

 signated Pipes or Fissures according to their form. 



The section of every pipe shows variation of contour, 

 sometimes considerable, at different depths ; the walls 

 of most pipes exhibit in consequence frequent hollows 

 and protuberances. A bodily deflection of the pipe is 

 not uncommon, e.g., the Kimberley and Newlands 

 Mines, while in the St. Augustine's Mine, at Kimberley, 

 the pipe has a twisted course. The Smith- Welte- 

 vreden Mine in the Harts River Valley has apparently 

 been formed by the coalescence of two pipes, for it is 

 dumb-bell shaped in plan. 



The relation between pipes and fissures is a very in- 

 timate one, and the majority of pipes have dyke-like 

 offshoots either at the surface or deeper down. It is 

 not uncommon to find that a pipe is of slightly later 

 age than the fissure on which it is situated; less fre- 

 quently dykes are found cutting across the material 

 filling a pipe. In either case the two kinds of rocks 

 show lithological differences. It has also been found 

 by experience that the material in dykes and fissures 

 rarely contains diamonds. 



1 T. G. Bonney, G. M., 1899, p. 309. 

 23* 



