358 KIMBERLITE 



Ecca and Beaufort beds, strata no longer found in this 

 part of the Colony. The blue-ground is thus seen to 

 be a very heterogeneous material varying in character 

 not only from one pipe to another, but in different por- 

 tions of the same pipe. 



A very peculiar property of the blue-ground is its 

 tendency to crumble and pulverise after having been 

 exposed to the action of moisture and the atmosphere 

 for a short period. Certain portions, however, for some 

 unexplained reason refuse to disintegrate, and have 

 been given the name of " hard-blue " or " hardibank " ; 

 the constituent minerals in a hardibank are as a rule 

 much less altered than in soft blue-ground. Hardi- 

 bank is very much more common in fissures than in 

 pipes, and this has led to the distinction of " fissure- 

 kimberlite " from " pipe-kimberlite," employing the 

 term kimberlite in its widest sense. 



It is usually believed that the breccia now filling the 

 pipes was intruded in the condition of a stiff mud, but 

 the question of the temperature of the material at the 

 time of its eruption is one that has given rise to a cer- 

 tain amount of controversy. It is certainly true that 

 in the great majority of the pipes there is little or no 

 sign of metamorphism either of their walls or of the in- 

 clusions in the blue-ground ; on the other hand the 

 effect of heat is very pronounced in other pipes, e.g., 

 Kimberley West and Paardeberg East, twenty-eight 

 miles south-west of Kimberley. 



It seerns probable that the material forming kimber- 

 lite, at one time in a highly heated state, became rapidly 

 chilled in its ascent in the pipes, so that different mines 



