48 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. [MEM0IE f^ A L TI xix? 



that they are entirely unconnected with the Bushveld plutonic complex, with which they are so 

 closely associated. Bowen's ('15) conception of the origin of alkaline rocks from a normal 

 magma by extreme gravitational differentiation would suggest that they might have formed 

 the last phase of Bushveld igneous activity except for the difficulty that they do not rest di- 

 rectly upon the granites only, but also upon the norites, and the very long duration of plutonic 

 activity involved. Smyth's ('13) hypothesis, on the other hand, the application of which is 

 open to a similar objection, might lead us to regard gaseous concentration of alkalies into the 

 residual magma of the Bushveld complex, as the essential mechanism of differentiation pro- 

 ducing the alkaline series of igneous rocks, in support of which may be cited Brouwer's ('17) 

 emphasis upon the abundance of fluorine in the rocks. Daly ('14) again would look to an 

 interaction between the magna and the dolomite beneath the Bushveld complex as a probable 

 factor in the genesis of the Pilandsberg rocks while Shand ('22) has described his hypothesis of 

 the origin of the alkaline rocks as "a synthesis of the views of Bowen, Smyth, and Daly." 



Possibly the next in age of the basic intrusive rocks in South Africa is the great dike of 

 Southern Rhodesia described by Mennell ('10), Zealley ('11, '15), and Wagner ('14). It is 300 

 miles in length and about 4 miles in width, and is composed of rocks resembling the basic rocks 

 of the Bushveld complex. It invades the older granite and schists of the Swaziland series, 

 especially following the margin of the two. It is a highly differentiated mass composed of 

 felspathic norite, enstatite-rock and harzbergite and occasionally dunite, all forming elongated 

 strips or wedges, lying parallel to the edge of the intrusion, the less basic and finer grained por- 

 tions occurring in the interior. The rock is, however, not foliated, though schlieren-masses of 

 chromite are present. Small bosses and dikes of granite occur, never passing beyond the basic 

 mass, also aplitic and pegmatitic veins, quartz-veins, and those composed solely of acid plagio- 

 clase. Mennell thinks this mass an intrusion into a gently inclined thrust-plane; Wagner holds 

 it to be a very elongated asymmetric laccolite. Zealley believes it to be a dike. The gentle 

 dip of the bands of the differentiates and of the schlieren-structure urged in support of the first 

 two hypotheses, Zealley thinks explicable on other grounds. 



The next great series of intrusions are of an age between the Jurassic beds of the upper 

 portions of the Karroo series and the Upper Cretaceous Umtavuna beds in which they occur as 

 bowlders. They invade the whole immense area of the unfolded Karroo sediments but are 

 absent from the folded Karroo rocks of southern Cape Colony. The rocks are dolerites, forming 

 sills and dikes, the latter clearly the feeding channels of the former. They are for the most part 

 ordinary dolerite of a purely calcic character, but olivine dolerite is not infrequent. Acid veins 

 of a oranophyric character are by no means uncommon and are generally a few inches only in 

 width. These intrusions run for very long distances concordantly in the stratification of the 

 sediments but sometimes traverse them obliquely either with gentle or steep inclination, though 

 the invaded beds may be quite horizontal. In the Queenstown district there is a great tendency 

 for a single intrusion to form a highly undulating sheet in horizontal strata which on being de- 

 nuded gives rise to annular outcrops or pseudo-laccolitic masses (Du Toit '05). The regularity 

 of the intrusions may be broken by numerous splits and offshoots from the main mass, while 

 new sills make their appearance in the strata farther away from the main intrusion, these being 

 generally connected with the latter either directly or underground, so that the mass of sedi- 

 ments is penetrated by a plexus of sheets in the most complicated manner, but without affect- 

 in<* appreciably the dip of the strata. A remarkable series of intrusions occur in Griqualand, 

 Pondoland, and Natal, which, though doubtless derived from the same magma as the dolerites, 

 develop much thicker masses, and are plutonic in petrographic character. The Insizwa mass 

 forms a huge dike of gabbro and norite which spreads as it rises and gives off on either side sev- 

 eral flatter and diverging sheets thus forming a roughly fan-shaped mass in which the metamor- 

 phosed sediments occur between the inclined sheets of igneous rocks. (See fig. 15.) These are 

 sometimes undulating, forming pseudo-anticlines and synclines in the horizontal mudstones 

 (Du Toit '10)." The Tabankulu mass adjacent to the latter shows the presence of gravitational 



« See also Goodchild ('17). 



