AC4DBMT0FSCHSNCBS] AFRICA. 47 



grade into gabbro. Bands of magnetite, and to a less extent of chromite, occur in the norite 

 and pyroxenite, running parallel with the general foliation, and are especially marked on the 

 eastern margin of the complex, where they may be associated with a narrow underlying zone 

 of highly felspathic norite, grading down into the normal rock (Hall '05, '09, Humphrey '11). 



The granite lies above the norite, the contact zone being sometimes so distinct that hills 

 of granite isolated by circumdenudation are seen to rest upon the norite (Hall '10). The rela- 

 tionship of the granite to the norite is generally an intrusive one, a contact-zone of norites 

 veined by granite having been seen in a number of cases, (Kynaston '08, Hall '09, Humphrey 

 '11, Wagner '12), though there is not necessarily any great age difference between the two. 

 Along the southern margin there appear regions where it is claimed that a continuous passage 

 between the two rocks occurs (Kynaston '08, Hall '03). In some cases the description of the 

 supposed passage-rocks strongly recalls the characters of the marscoite hybrids of gabbro and 

 granite occurring in Skye. 3 The lowest portions of the granite show very constantly a narrow 

 zone of ultra-acid rock which has sometimes been mistaken for an included mass of quartzite. 

 Hall ('10) states that its general dip is parallel to the schistosity of the underlying norites and 

 of the Pretoria series. It consists of coarsely granular quartz with more or less mica and grades 

 into a type containing orthoclase. Biotite, magnetite, ilmenite, and rutile are occasional accesso- 

 ries. Setting aside the suggestion that the rock may be of the nature of greisen, he holds that 

 it resulted from differentiation in the marginal portions of the granite adjacent to the norites, 

 where its power of differentiation was retained for the greatest length of time, having been 

 elsewhere restricted by the increasing viscosity of the magna due to increasing acidity and 

 decreasing temperature. Hall ('13) also describes a remarkable group of quartz-augite-mon- 

 zonite porphyries, representing a "differentiation of the granite magma, resulting in a thin basal 

 layer of less acid types which, in contact with the cooler floor of norite and other more basic 

 rocks, assumed a finer texture." 



The "red granite" above the basic zone is thus by no means a uniform mass. Locally 

 granophyres of comparatively fine grain occur in it, and Mellor ('08) and Kynaston ('09) show 

 that further complexity appears when we attempt to determine the relationship of the granites 

 to the Waterberg system. It would seem that several intrusions occurred, derived from the 

 same source. Bowlders of tourmalinized rocks and of granites in the Upper Waterberg con- 

 glomerates, point to the intrusion of granites into the Lower Waterberg rocks and their subse- 

 quent exposure by denudation in pre-Upper Waterberg times, but the invasion of Upper Water- 

 berg sediments by further masses of red granite similar to that of the rest of the complex shows 

 the continuation of the plutonic activity. 3 The invasion of the granites was followed in some 

 places by a further development of diabase sheets in the granite and Waterberg sandstones 

 which tend to grade into more acid felsitic rocks (Mellor '03) . Felsitic and allied volcanic rocks 

 are intercalated with the lower shales, etc., of the Waterberg series, and their possible magmatic 

 relationship to the Bushveld plutonic rocks has been variously discussed. 



The alkaline rocks which occur especially abundantly in the Pilandsberg, but also in a 

 number of other areas near the margin of the norite and gabbros, were originally supposed to 

 form an intermediate zone of differentiation (Hatch and Corstophine '05) . The work of Brouwer 

 ('10, '17) and Humphrey ('11) proved that the Pilandsberg rocks were the plutonic core of a 

 series of alkaline volcanic rocks resting directly on an eroded surface of the norites and granite 

 without any intervening Waterberg sediments. They are therefore of a date subsequent to 

 the erosion of the Waterberg sandstones and the exposure of the Bushveld plutonic complex. 

 There is no evidence to determine their age relative to the Karroo system, but Humphrey ('11) 

 thinks they are probably much newer than the Waterberg system. Wagner ('12), on the other 

 hand, points to the lengthy succession of granitic rocks from the Bushveld, some of an age sub- 

 sequent to the exposure of the earlier granites of the complex, and holds it premature to decide 



•E.g., "The transition from coarse-grained norite to the more fine-grained reddish granite is complete in a distance of about 30 to 40 yards. The 

 granite is mottled with green ferromagnesian constituents which are seen to be made up of a little augite together with yery much corroded horn- 

 blende" (Hall '03, pp. 35-37; cf. Harker '04). 



» Attention should be called to this recurrence of plutonic activity, after its apparent cessation and the intervening of a period of sedi- 

 mentation. An analogous feature is seen in New South Wales (p. 38) and perhaps in Scotland (p. 12). See postscript, p. 78. 



