January 9, 1908J 



NA TURE 



225 



volcanic peridotite of basaltic structure, or, according 

 to Rosenbusch's nomenclature, the palceo-volcanic re- 

 presentative of a biolite-bronzite-dunite, being an 

 olivine-bronzite-picrite rich in biotite." To this rock 

 he gave the name of kimberlite, distinguishing be- 

 tween kimberlite proper, a typical porphyritic lava ; 

 litinbcrlite breccia, the same lava broken and 

 crushed by volcanic movements within the pipe, and 

 crowded with included fragments of foreign rock ; 

 and kimberlite tuff, the fragmental and tufaceous 

 portion of the same volcanic rock, all these varieties 

 passing into one another and occurring together in the 

 same neck or crater. The kimberlite breccia consti- 

 tutes, according to Carvill Lewis, the greater portion of 

 the pipe material, but it is traversed by dykes of 

 kimberlite proper, and contains streaks and patches 

 of a soft, " soapy " material which appears to be 

 kimberlite tuff. As to the diamond, it is as much a 

 constituent of kimberlite as the inore commonly 

 occurring biotite, garnet, titanic and chromic iron 

 and perovskite, and consequently must have been 

 produced in situ. 



From this view Bonney was led to dissent by 

 a inicroscopic examination, made in .1899, of speci- 

 mens from the Ncwlands mines in West Griqua- 

 land (" The Parent Rock of the Diamond in 

 .South Africa," Proc. Roy. Soc, vol. Ixv., 1899, 

 p 620). Among these specimens was a rounded 

 boulder of a rock which was described bv him as a 

 " holocrystalline mixture of chrome-diopside and 

 garnet, with a few small enclosures of olivine, in 

 other words, a variety of ecklogite and of igneous 

 origin." The interest of the specimen lay in the 

 fact that it contained diamonds, and from this Bonnev 

 drew the momentous conclusion that the birth-place 

 of the diamond and of the garnet, pyroxenes, olivine, 

 &:c.. was not the " blue ground " itself, but the 

 ecklogite which occurred in the latter as foreign 

 boulders. He further concluded, from the smooth e.\- 

 terior of the specimen examined, that the boulders 

 were water-worn, and were derived from a conglom- 

 erate bed "at the base of the sedimentary series in 

 proximity to a crystalline floor." The blue ground 

 then, according to Bonney, is a true breccia produced 

 by the destruction of both crystalline and sedimentary 

 rocks, the " result of shattering explosions followed 

 by solfataric action." 



R. Beck {Zeits. fiir prakt. Geol., 1879, P- 4'7)i t° 

 whom similar specimens from the Newlands mines had 

 been sent, differed from Bonney as to the origin of the 

 so-called boulders of ecklogite, which he considered to 

 be concretions formed at great depth in the kimberlite 

 magma — like the well-known olivine nodules in the 

 basalt of Finkenberg, near Bonn, described by Rosen- 

 busch and others as early segregations from the basalt 

 magma. The serpentine breccia which represents 

 the present condition of the kimberlite magma con- 

 tains all the constituents of the nodules. The round- 

 ing of the boulders is to be ascribed to attrition during 

 the upward course of the pyroclastic material in the 

 pipe. Beck agreed with Bonney that the diamonds 

 were formed at great depth, but for a different reason, 

 namely, that only at great depths could such coarse- 

 grained granular segregations from the magma have 

 been produced. In a more recent paper (Zeits. der 

 tieiilscb. geol. Gesellsch., 1907, p. 226), Beck gives re- 

 sults of a further examination of the Newlands eck- 

 logite, which, by the way, he proposes to call 

 " Griquaite." In addition to the constituents already 

 named, he finds biotite, perovskite, zircon, rutile, and 

 graphite. He repeats his conclusion that the 

 " boulders " are deep-seated (intra-telluric) segre- 

 gations from the same magma from which the pipe 

 material is derived, and ascribes the genesis of the 



NO. 1993, VOL. 77] 



diamond to the presence in the original magma of 

 metallic carbides, it being his opinion that not only 

 the diamonds contained in the ecklogite, but also the 

 isolated individuals in the blue ground, must have been 

 formed at great depth. 



The views of the earlier writers can be grouped 

 into two distinct theories : — (i) The pipe material is a 

 breccia or tuff produced with its contained diamonds, 

 by violent volcanic agencies, from pre-existing rocks 

 (Cohen, Chaper, Bonney, Beck). (2) It is a true 

 eruptive lava which has solidified in situ, the dia- 

 monds having been separated out during this consoli- 

 dation (Dunn, Maskelyne, Flight, Carvill Lewis). 



Among the recent writers on the subject, H. S. 

 Harger and G. S. Corstorphtne support the view that 

 the diamonds have been derived trom a deep-seated 

 source, the former mentioning that out of a parcel of 

 372 stones examined by him, 1 19 were found to be 

 broken fragments of original crystals. F. W. Voit, 

 oa the other hand, regards the " splints " as either 

 imperfectly formed crystals or as having been broken 

 during the processes of niining and washing. (He 

 w-ould find it difiicult, however, to explain the frac- 

 tured character of the Cullinan diamond ' by either 

 of these hypotheses.) Voit regards the pipes as em- 

 bryonic volcanoes. According to him, the igneous 

 magma which rose in the pipes, being unable to 

 reach the surface, was forced to solidify under 

 the pressure of the superincumbent strata in circular 

 or elongated moulds, thus giving rise to what he 

 terms "conical batholites." While the extruding 

 magma was still in the semi-plastic state, it was 

 agitated by further gaseous eruptions, to the action 

 of which the brecciated character of the blue ground 

 is largely to be ascribed. He regards the ecklogite 

 " boulders " as concretions of the kimberlite magma, 

 formed in situ. He admits, however, the presence 

 in the blue ground of boulders of granite, gneiss and 

 ciystalline schists of e.xotic origin. 



.\. L. du Toit, of the Cape Geological Commission, 

 who during 1906 was working in the Kimberley dis- 

 trict, has formulated a theory which combines parts of 

 both the earlier views, and is more in accord with 

 modern ideas of petrogenesis. According to him, 

 kimberlite is a hybrid rock derived from a deep-seated 

 magma (having the composition of a limburgite) 

 which Incorporated the shattered fragments of the 

 various holocrvstalline basic and ultra-basic com- 

 ponents of the floor through which it broke. During 

 its further ascent the eruptive mass caught up and 

 included in its body fragments of the sedimentary beds 

 through which it passed. .\ portion of its brecciated 

 character, however, is to be attributed to movements 

 during consolidation. Of the minerals found in the 

 blue ground, du Toit considers the tremolite, smarag- 

 dite, epidote, orthite, tourmaline, muscovite, biotite, 

 apatite and zircon to be xenocrysts derived from deep- 

 seated rocks of acid composition (granite, gneiss, 

 pegmatite, &c.); while he regards the olivine, 

 pyroxenes, garnet, ilmenite, magnetite, chromite, 

 spinel, sphene. kyanite, and the diamond as xenocrysts 

 derived from basic and ultra-basic holocrystalline 

 rocks. In addition, the original eruptive magma 

 gave rise on consolidation to authigenic crystals of 

 olivine, diopside, brown mica, magnetite, ilmenite. 

 chromite, apatite, perovskite, nepheline and melilite. 



In support of his view, du Toit gives instances 

 of strong contact metamorphism effected by the 

 pipe material during its intrusion. Few cases of 

 contact-metamorphism, however, have hitherto been 

 observed. Corstorphine (loc. cit., p. 65), for instance, 



1 riWe F. H. Hatch anrt G. S. Corstorphii 

 Diamond recently found in the P/emier R 

 p. 170, 1905). 



, " A Description of the Big 

 le, Transvaal {GeoL Ma^., 



