C. B. Honcood ^ A. Wade— The Old Granites of Africa. 497 



Although only one undoubted matrix is known in Australia, the 

 ■wide distribution of diamonds in the alluvial deposits suggests that 

 many others are present. The erosion of a dolerite dyke must of 

 necessity be slow, and seems inadequate to supply the quantity 

 already recovered from the gravels. Volcanic agglomerates seem more 

 suited to explain this wide distribution, from their easy destruction 

 by surface agencies. Moreover, in such aggregates diamonds are 

 more likely to be plentiful, if one may argue from the distribution of 

 other minerals of homogeneous nodules. For instance, both at 

 Kakanui and Elie Ness large hornblende crystals may occasionally be 

 found in the dykes traversing the agglomerates, but they are much 

 more abundant by themselves as fi'agments in the agglomerate, and 

 this is true of the minei'als of most crystalline tuffs. 



VII. — The Old Geanites of the Transvaal and of South and 

 Central Afkica, by Cuthbert Baring Horwood, A.E.S.M., Assoc. 

 M.Inst.C.E., F.G.S., with a Petrographical Description of the 

 Orange Grove Occitrrence, by Arthur Wade, B.Sc. (Lend.), 

 A.R.C.S., F.G.S. 



{Continued from the October JSfumher, p. 468.) 



"11 IE. J. HAYS HAMMOND in his paper, "Gold-mining in the 

 JjX Transvaal, South Africa," read at the Richmond meeting of 

 the American Institute of Mining Engineers in February, 1901, 

 referring to this Old Granite, states: "There are at places intrusions 

 from this enveloping granitic mass into the younger formations." 



Mr. Dorffel called attention to the fact that it had never been proved 

 to be intrusive in the Witwatersrand System, and expressed his opinion 

 that it Avas not, although this assumed the existence of an Archaean 

 formation, which apparently was wanting in the Transvaal yet existed 

 in Cape Colony as the Malmesbury Beds. 



In January of the following year, in supporting Mr. Dorffel's views, 

 I showed ^ that in the case of the Orange Grove occurrence all the 

 evidence indicated that there the granite is not intrusive in the 

 Witwatersrand System. - 



In March of the same year Dr. Corstorphine^ described the exposure 

 of Witwatersrand quartzite resting directly on the Old Granite on the 

 farm Uitkyk in the Heidelberg district, where no veins are to be 

 seen passing upward into it from the granite, and in the closest 

 proximity to the granite the quartzite presents the same petrographical 

 characteristics as elsewhere. AVhere the surface of the granite can 

 be seen below the escarpment it gives the impression of a rounded 

 and worn mass. From the evidence afforded from this occurrence he 



' "Contribution to the Discussion on Mr. Dorffel's Paper, 'Note on the 

 Geological Position of the Basement Granite,'" by C. B. Horwood: Trans. Geol. 

 Soc. S.A., vol. vi, pt. vi, pp. 114, 115. 



- The Witwatersrand System consists of the Upper and Lower Witwatersrand 

 Beds ; this division is purely an arbitrary one, and is placed at the base of the Main 

 Reef Series. 



^ "The Geological Relation of the Old Granite to the Witwatersrand Series," 

 by G. S. Corstorphine : Trans. Geol. Soc. S.A., vol. vii, pt. i, pp. 9-12. 



decade v. VOL. VI. NO. XI. 32 



