320 Professor T. G. Bonney — Parent-rock of the Diamond. 



more basic, would prove to be the birthplace of the diamond. Can 

 it possibly be a derivative mineral, even in the eclogite ? Had 

 it already crystallised out of a more basic magma,^ which, 

 however, was still molten, when one more acid was injected, and 

 the mixture became such as to form eclogite ? But I content myself 

 with indicating a difficulty, and suggesting a possibility ; the fact 

 itself is indisputable : that the diamond occurs, though rather 

 spoi'adically, as a constituent of an eclogite, which rock, according 

 to the ordinary rules of inference, must be regarded as its birthplace. 

 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 

 described 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 explosive 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 crystalline I'ocks.^ The 

 ' washings,' ^ a pai'cel of which I received from Mr. Trubenbach, also 

 show that the boulders are really water- worn. Besides two unworn 

 pieces of pyrite and a rough bit of eclogite, about three-quarters 

 of an inch in diameter, the pyroxenic constituent of which was 

 a bright emerald green (? smaragdite), I find part of a subangular 

 fragment of chrome-diopside associated with two or three flakes 

 of the usual mica, a well-rounded garnet fully 06 inch across, and 

 half a well-worn pebble of eclogite, about one inch long and half 

 an inch thick. The rounded water-worn look of the great majoi'ity 

 of the smaller constituents (chiefly garnets and pyroxenes), about 

 the size of hemp-seed, is very obvious. I had suspected some of 

 the grains washings fi-om the De Beers Mine to have been similarly 

 treated ; but here it is indubitable, indeed many of the dark green 

 specimens ai'e so smooth outside tliat they could only be identified 

 after fracture. The ordinary diopside can, however, be recognised, 

 with some of a clearer and brighter green. Most of the garnets 

 are pyropes, but a few resemble essonite. I find also some grains 

 of iron oxide and of vein quai'tz. Thus, the presence of water- worn 

 fragments, large and small, in considerable abundance, shows the 

 ' blue ground 'to be a true breccia, produced by the destruction of 



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

 contain much of that constituent. 



2 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 from 

 which these boulders were taken (at various depths, from nearly 100 to about 300 

 feet) cannot be any alluvial deposit, but is the typical 'blue ground,' practically 

 identical with that in the Kimberley mines. 



^ The name is given to the mineral residue left after washing away the deoomposed 

 matrix of the ' blue ground.' 



