630 ORVILLE A. DERBY 



tive rock, composed largely of olivine, would almost inevitably 

 result. 



We thus have in the formative stages of the pipe-fillings (or at 

 least in early stages of their history) a sufficient agency for the 

 hydration of their eruptive portions. Such a change has been 

 observed down to such extraordinary depths that the usual explana- 

 tion of atmospheric weathering seems utterly incredible. 1 The 

 hydration, which to a greater or less degree seems to be character- 

 istic of all known occurrences of undoubted kimberlite, whether 

 appearing in pipes or dikes, is accompanied by the formation of a 

 certain amount of calcite, which involves the introduction, in some 

 stage of the history of the rock, of carbon in a condition to form the 

 carbonic acid locked up in the mineral. This introduction may also 

 be most plausibly assigned to the stage of thermal agitation, of 

 which the fragmenting and explosive actions were the climax. 

 The analysis cited in the preceding note, representing the least- 

 altered kimberlite thus far examined, gives 2 . 54 per cent of carbonic 

 acid, corresponding to about 5,000 grams of pure carbon to the 

 unit of volume (load =0.453 cubic meters) used by the African 

 miners in measuring their material. This amount of carbon, 

 if present in the form of diamond, would give about 25,000 carats, 

 whereas the usual yield of the De Beers load is under 1 carat 

 (1/5 gram). 



There are thus strong a priori reasons for attributing to deep- 

 seated causes long since extinct a great part of the hydration and 

 carbonation which the eruptive rock, originally free from water 

 carbon, has suffered. If such was the case, it becomes important 

 to distinguish the deep-seated actions from those of the atmosphere, 

 which, acting from above downward, have long been producing 

 similar results. These superficial results would be superimposed on 

 the pre-existing ones, if such existed, down to a certain depth. 

 No question can be raised regarding the correctness of the view, 



1 Dr. Wagner gives an analysis of a specimen of kimberlite collected in the deepest 

 part (2,040 feet from the surface) of the De Beers mine, which had 6.81 per cent of 

 combined water. This, as he expressly states, represents the best-preserved material 

 to be found in the Kimberley group of mines, although in the neighboring Kimberley 

 mine the pipe has been opened up nearly 1,000 feet farther down, or fully 3,000 feet 

 from the surface. From this it may legitimately be inferred that there is little likeli- 

 hood of finding unhydrated kimberlite in the known South African diamond mines. 





