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Many thin sills and some dikes younger than almost all the associated normal Karroo dolerites occur in a narrow belt that extends from Durban along coastal Natal into Zululand where it flanks Stormberg (Lebombo) volcanic rocks on the west. Characteristic features of these intrusions in the field are glassy and variolitic margins, abundant minute amygdales, and conspicuous xenoliths of typical Natal Precambrian granite and quartzo-feldspathic gneiss and schist.

They contain more silica and potash, less iron and lime, and much less magnesia than average Karroo dolerite. Petrographically they are all very similar. The holocrystalline rock has plagioclase ranging from some labradorite to much andesine-albite associated with prismatic clinopyroxene, titanomagnetite plates, much fine quartz-feldspar micrographic intergrowth, and xenocrysts mainly of quartz fragments. The chemical composition spans the range from rhyodacite to trachyandesite.

The Effingham rock type is considered to have been produced from the melting and subsequent assimilation of sialic rocks by a slightly differentiated, volatile, and iron-enriched magma at depth. Heat energy supplied by a thermal convective process contributed to the formation of the new magma which was injected in the later phases of Karroo magmatism, during downwarping of what is now coastal region, and probably just prior to continental disruption.

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