The sedimentary rocks of the Pretoria Series above the Magalietberg quartzite consist mainly of quartzite and hornfels. The stratigraphical subdivision of these rocks is based on the principle that a cyclothem begins with coarse-grained material like sandstone which then passes into fine-grained sediments like shale. Accordingly, the hornfels above tile Magaliesberg quartzite should not be regarded as the start of a new cyelothem but should be grouped in the same zone as the Magaliesberg quartzite. Consequently, the Magaliesberg Stage is subdivided into the Majn Magaliesberg and the Lakensvlei zones, which consist of the Magaliesberg quartzite and hornfels and the Lakensvlei quartzite and hornfels respectively. The Smelterskop Stage is divided into the Steenkampsberg and the Dullstroom zones. The former consists of quartzite and the latter of andesite. The Steenkampsberg quartzite, which forms the "floor" of the Bushveld Complex is considerably recrystallized, and has a sugary texture. Sillima.nite, plagioclase and muscovite are usually present in this quartzite layer. The Magaliesberg and Lakensvlei quartzite is slightly recrystallized, and chlorite and epidote are prominent constituents. According to petrofabric analyses of the quartzite layers, it is evident that only the Steenkampsberg quartzite, having 9% concentration of the c-axes of the quartz grains, was affected by the thermal metamorphism which accompanied the emplacement of the Bushveld Complex. Two layers of black sand in the Steenkampsberg quartzite represent a fossil beach sand, which was deposited in the littoral zone. The beach sand consisted originally of ilmeno-magnetite, zircor., magnetite miilmenite, and during metamorphism the magnetite and ilmenite were altered to hematite and a mixture of ilmenite and rutile respectively. The Dullstroom volcanics on Houtenbek contain many inclusions of quartzite and zonary amygdales. Plagioclase and orthopyroxene are the main constituents of the lava and they indicate that the Dullstroom lava was metamorphosed under conditions corresponding to the pyroxene hornfels facies. Limestone layers, interlayered in the hornfels above the Magaliesberg quartzite, have been altered either to a diopsidefels or to a marble. Two types of aluminous metamorphic products could be distinguished, viz. cordierite hornfels and Imlscovite-biotite shale. Both formed in the hornblende hornfels facies and the difference in their mineral assemblages should be ascribed to the differences in chemical composition. The cordierite hornfels contains more mg2+ and A13+ and relatively less K+ than the muscovite-biotite shale. According to the sedimentology of the sediments of the Pretoria Series, the quartzite could have formed in a littoral environment, whereas the hornfels could have been deposited by the action of turbidity currents under neritic conditions. Three new chemical analyses of the rocks intrusive into the Pretoria Series are supplied, and according to both the chemical and mineralogical composition these rocks can be grouped into an older undifferentiated and a younger differentiated group. The former usually consists of diorite and the latter comprises a glass bearing margin and a differentiated central portion of the sills. The latter consists of harzburgite, dolerite, pyroxenite and quartz dolerite. The rocks of the differentiated suite have been uralitized to a variable extent and can be called diabase. They belong to the hornblende hornfels facies. The diorite on the other hand was not affected, because hornblende and plagioclase are stable in the hornblende hornfels facies. The composition of the glassy dolerite evidently represents the composition of the original magma which formed the differentiated sills. The chemical composition of the residual glass of the glassy dolerite indicates that iron enrichment characterizes an early stage of differentiation of a tholeiitic magma. The rocks of the layered sequence of the Bushveld Complex consist of norite, hyperite, chromitite and a marginal zone. Three types of marginal rocks, viz. micro-norite, quartz norite and quartz micro-norite, could be distinguished. The micro-norite forms a continuous rim along the margin of the Bushveld Complex and it represents a chilled margin. The quartz norite is a hybrid rock, which has assimilated sedimentary material. The quartz micro-norite replaces the micro-norite where assimilation has preceded the rapid chilling of the layered sequence of the Bushveld Complex. The two chromitite seams on De Kafferskraal and Sterkfontein belong to the Upper Group of chromitite seams. According to the petrography of the chromitite it is evident that the chromite grains settled from a magma rather than crystallized from a late magmatic residuum. On De Kafferskraal the chromitite seams end abruptly against the Steenkampsberg quartzite, without any evidence of faulting along the contact between the Bushveld Complex and the Pretoria Series. The decrease in the width of outcrop of th0 Steenkampsberg quartzite on Kliprivier, Wanhoop, De Kafferskraal and Verloren Valei is explained by the transgression of the rocks of the layered sequence of the Bushveld Complex across the sedimentary rocks of the Pretoria Series. The slight folding on De Kafferskraal is insufficient to explain the decrease of 3,000 ft. in the width of outcrop of this quartzite layer and consequently this rather sudden and phenomenal change in width of igneous material is explained by transgression.

Dissertation (MSc)--University of Pretoria, 1967.