502 C. B. Hom-ood ^ A. Wade— The Old Granites of Africa. 



they, especially the hornblende, being bent, and even broken, while 

 minute fault-planes run right across the cleavage. Among the 

 various varieties he mentions the occurrence of corderite- gneiss, and 

 also directs attention to the frequent occurrence of highly-crystalline 

 limestone as well-defined layers in the gneisses,^ and of thin beds 

 of haematite and magnetite, which are often interbedded with the 

 gneisses. 



He lays stress on the fact that geologists who have made a special 

 study of the origin of gneisses are inclined to look upon them as 

 being of magmatic origin, as being, in fact, original granite foliated 

 and turned into gneissoid rocks by enormous pressure caused by the 

 contraction of the earth's crust. Also that northern geologists who 

 have studied the old crystalline schists assume the existence of 

 a granite older than all crystalline schists — forming the basement 

 whereon the geological formations began to be built up, whilst they, 

 during a long period of denudation, furnished by their detritus the 

 material from which the sedimentary rocks could be formed. Two 

 years later he states ^ that in the Limpopo district granite resembling 

 that which is intrusive in the Swaziland Series also occurs and 

 is intrusive in the gneiss formation, and that its intrusive nature 

 can often be observed along the steep banks of the Limpopo River. 

 Also that inclusions of crystalline, well-foliated rocks, such as 

 coarsely-crystalline well-foliated hoi'nblende rock containing very 

 little felspar, augen-gneiss, etc., which certainly do not belong to 

 the Swaziland Series, and which are apparently in no way related to 

 the granite, occur as metamorphosed masses in the Old, intrusive, 

 Granite. In the Vryheid district, for example, he found such a rock 

 and also augen-gneiss occurring as metamorphosed masses in the Old 

 Granite, which had evidently been caught up in it at the time of 

 its intrusion. Rounded masses of hornblendic and augen-gneisses 

 also occur in the granite between Melmoth and Eshowe in Zululand.^ 



In this connexion I might mention, that dark inclusions consisting 

 of amphibolite or closely allied rocks, occur abundantly in the granite- 

 gneiss of the Laurentian System of Eastern Canada, and that there 

 is positive proof that some, and possibly all, of these are remnants 

 of the walls or roof of the granite bathyliths which have fallen into 

 the granite-magma, and have partaken of its subsequent movements.* 

 Also the black inclusions in the granite-gneisses of the Adirondacks 

 are considered to be broken masses of an older rock caught up by 

 the granite-gneiss when the latter was still in a molten condition.* 



1 He gives an occurrence of calcite rock, which is interbedded between the schists 

 near Zand River, as an example of a highly altered igneous rock. Probably the 

 crystalline limestones in the gneisses have also originated from the alteration of 

 igneous rocks. ' 



- " Gneiss Formation in Africa," by Dr. F. W. Voit: Trans. Geol. Soc. S.A., 

 vol. X, pp. 90-4. 



3 Loc. cit., p. 93. 



* " On the Structure and Relations of the Laurentian System in Eastern Canada, " 

 by Frank Dawscn Adams, D.Sc, F.R. S. : Quart. Journ. Geol. Soc, vol. Ixiv, 

 No. ccliv, pp. 133-5. 



= "Report on the Crystalline Rocks of St. Lawrence Countv," by C.H. Smyth, jun.: 

 N.Y. State Mus. 49th Ann. Rep., 1895, vol. ii (1898), p. 490. 



