The Precambrian Malvernian complex of the Malvern Hills and the Johnston complex of south Pembrokeshire contain basic to acid igneous rock of similar chemical composition and of calc-alkaline type. Both complexes contain minor rock types of hybrid origin and rock which is enriched in potash. The most important difference between the two complexes is the potash-poor nature of the Johnston and the potash-rich nature of the Malvernian igneous rock. This difference may be explained by the position of the complexes in relation to the contemporaneous orogenic activity suggestive of Precambrian subduction.

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