The Parent-rock of the Diamond in South Africa. 235 



Conclusion. 



Thus the diamond has been traced up to an igneous rock. The 

 " blue ground " is not the birthplace either of it or of the garnets, 

 pyroxenes, olivine, and other minerals, more or less fragmental, which 

 it incorporates. The diamond is a constituent of the eclogite, just as 

 much as a zircon may be a constituent of a granite or a syenite. Its 

 regular form suggests not only that it was the first mineral to crystal- 

 lise in the magma, but also a further possibility. Though the occur- 

 rence of diamonds in rocks with a high percentage of silica (itacolumite, 

 granite, &c.) has been asserted, the statement needs corroboration. 

 This form of crystallised carbon hitherto has been found only in 

 meteoric iron (Canyon Diablo),* and has been produced artificially by 

 Moissan and others with the same metal as matrix. But in eclogite 

 the silica percentage is at least as high as in dolerite • hence it is diffi- 

 cult to understand how so small an amount of carbon escaped oxidation. 

 I had always expected that a peridotite (as supposed by Professor 

 Lewis), if not a material yet more basic, would prove to be the birth- 

 place of the diamond. Can it possibly be a derivative mineral, even 

 in the eclogite ? Had it already crystallised out of a more basic 

 magma,f which, however, was still molten, when one more acid was 

 injected, and the mixture became such as to form eclogite 1 But I 

 content myself with indicating a difficulty, and suggesting a possi- 

 bility ; the fact itself is indisputable : that the diamond occurs, though 

 rather sporadically, as a constituent of an eclogite, which rock, accord- 

 ing to the ordinary rules of inference, must be regarded aw its birth- 

 place. 



This discovery closes another controversy, viz., that concerning the 

 nature of the "Hard blue" of the miners (Kimberlite of Professor 

 Lewis), in which the diamond is usually found. The boulders de- 

 scribed in this paper are truly water worn. The idea that they have 

 been rounded by a sort of " cup and ball " game played by a volcano 

 may be dismissed as practically impossible. Any such process would 

 take a long time, but the absence of true scoria implies that the explo- 

 sive phase was a brief one. They resemble stones which have travelled 

 for several miles down a mountain torrent, and must have been derived 

 from a coarse conglomerate, manufactured by either a strong stream or 

 the waves of a sea from fragments obtained from more ancient crystal- 

 line rocks.]; The "washings," a parcel of which I received from 



* P.S. — The Novo Urei meteorite (not wholly iron) was forgotten. 



f This, however, cannot have been very rich in iron, because diopside does not 

 contain much of that constituent. 



t As these eclogites are very coarsely crystalline, we are justified in assuming 

 they were once deep-seated rocks, and so much more ancient than the date of the 

 conglomerate. To prevent any misunderstanding I may repeat that the matrix 



