356 KIMBERLITE 



To the peculiar material filling the pipes and fissures, 

 commonly called "Blue-ground," Carvill Lewis 1 gave 

 the name Kimberlite ; he distinguished three varieties, 

 kimberlite proper, a porphyritic ultrabasic lava, kim- 

 berlite breccia, and kimberlite tuff, but these varieties 

 pass into one another by insensible gradations. In this 

 book the term kimberlite will be used to cover all these 

 kinds of rock, as indeed has been the general practice 

 of late years. It is difficult to describe the exact nature 

 of kimberlite, because the rocks, especially their ground 

 mass, have always been altered even in specimens ob- 

 tained from the deepest parts of the mines. 



While rocks are occasionally obtained that show un- 

 mistakable structures characteristic of lavas, such as 

 amygdales arranged in bands, the fragmental nature 

 of most of the material called kimberlite is equally 

 clear. It will be shown later that there is reason to be- 

 lieve that Carvill Lewis' kimberlite proper is very 

 closely allied to, if not identical with, melilite-basalt. 



The rock as obtained from deep workings has a dark 

 slaty-blue or bluish-green colour, and from that char- 

 acter it got the name " Blue-ground " ; near the surface, 

 where it has been more weathered, it has a yellowish- 

 brown tint, and in this condition is termed " Yellow- 

 ground ". The depth at which the yellow gives place 

 to the blue ground varies greatly according to the 

 nature of the rock and other circumstances. 



Kimberlite is generally a serpentinous breccia contain- 

 ing fragments of various minerals, the chief of which 



1 Carvill Lewis, Papers and Notes on the Genesis and Matrix of t)ie 

 Diamond, London, 1807. 



