496 J. Allan Thomson — Diamond Matrices of Australia. 



in the blue ground may prove to be homogeneous inclusions of 

 volcanic rocks, which are analogous to, but hardly the same as the 

 basic secretions of plutonic rocks. ^ The apparent reason that this 

 explanation has not been earlier put forward must lie in the com- 

 parative rarity of garnetiferous homogeneous inclusions. A brief 

 survey of the known examples may be of interest. Lacroix describes 

 a few occurring in alkaline rocks when these themselves contain 

 melanite.- Zirkel describes certain garnet-wollastonite nodules in the 

 basalts of the Rhine Valley, but explains them as inclusions of lime- 

 stone which have undergone thermal metamorphism by the action of 

 the magma (accidental xenoliths).^ These two kinds of garnet 

 nodules are hardly germane to this discussion, and there remain, 

 only three instances known to me which have a bearing on the 

 so-called eclogites, Zirkel in. the same paper* gives adequate reasons 

 for regarding other garnet-felspar-augite nodules as early segregations 

 from the basalt magma. I have described garnetiferous olivine, augite 

 and hornblende nodules in the limburgite breccia of Kakanui. Finally 

 it is almost certain that similar nodules occur in the necks of the 

 rife coast, for at Elie Ness, where hornblendic, augitic, biotitic, and 

 felspathic nodules are abundant, garnets may occasionally be picked 

 up in the agglomerate and on the beach. It is worthy of note that 

 the two latter occurrences are in the agglomerates of limburgites, 

 although felspathic basalts are present in both localities. From the 

 descriptions already given, the so-called eclogites of Ruby Hill and 

 Snodgrass appear to come under this category, and it is not improbable 

 that the other eclogite occurrences of New South Wales, cited by 

 Mr. Pittman,* may prove to be similar. 



The presence of chrome diopside and garnet in the Oakey Creek 

 dolerite is strongly suggestive of the occurrence of garnet-pyi'oxene 

 nodules in this rock. The occurrence would not then differ so greatly 

 from those of South Africa as has been supposed. The importance 

 of this case is that we have direct evidence of the nature of the 

 magma from which the nodules segregated. 



These views are largely conjectural, but this paper will serve its 

 purpose if it secures an adequate consideration of them when the full 

 history of the Australian diamond matrices comes to be written. 

 Should they be confirmed, instead of comparing the Australian 

 occurrences to the South African, the reverse process may take place, 

 and one may see in the diamond ' pipes ' of the latter continent the 

 necks of volcanic rocks such as limburgites, in which the accidental 

 xenoliths and homogeneous inclusions have escaped excessive alteration, 

 while the ultrabasic volcanic material has been completely serpentinised. 

 The discovery of necks of mellilite-basalt already points in this 

 direction.* 



' J. A. Thomson, "Inclusions in some Volcanic Rocks": Geol. Mag., 1907, 

 Dec. V, Vol. IV, p. 500. 



^ A. Lacroix, Zes Enclaves de-i Roches volcaniques , Maqon, 1893. 



^ F. Zirkel, " Uber Uraussclieidungen in rheinischen Basalten " : Abh. Konigl. 

 Sachs. Ges. d. Wiss., 1903, xxviii, p. 103. 



* Loc. cit., p. 497. ^ Loc. cit. 



* A. "W. Eogers & A. L. du Toit, "The Sutherland Volcanic Pipes and their 

 Eelationship to other Vents in South Africa " : Trans. S.A. PhU, Soc, 1904, xv, p. 61, 



