The Karroo dolerites represent the intrusive phase of the early Jurassic basalts which built up the Basutoland lava plateau. Those in the Union of South Africa are described.

Both basalts and dolerites are of tholeiitic magma type and are characterized by low Fe2O3 and alkalies. Most of the dolerites show little differentiation, but olivine rich types and acidic veins and schlieren are recorded. The differentiation trend was toward moderate iron enrichment. The magma was extremely active toward the associated sedimentary rocks causing widespread rheomorphic and syntectic phenomena. Many of the rocks previously described as “diorite” or “granophyre” were found to be transfused siltstones.

The dolerites occur chiefly as sills, dikes, and inclined sheets, but “bell-jar” intrusions occur near the focus of activity southwest of Basutoland.

The mechanism of intrusion is discussed. The authors believe that the Karroo dolerites were injected under conditions of tension following the buckling of the Samfrau geosyncline and represent only one phase of the pronounced igneous activity which spread over a large portion of Gondwanaland during Trias-Jura time.

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