New analytical data are presented to show that the Karroo basalts are divisible into two distinct provinces. A northern province includes all the Rhodesian basalt localities and is characterised by values of K, Ti, P, Ba, Sr and Zr which are abnormally high for tholeiitic rocks. The southern province includes the basalts of Basutoland and Swaziland, and the Karroo dolerites of South Africa, all of which have a normal tholeiitic geochemistry.
Petrographically the provinces may be distinguished by a different order of crystallisation. The rocks of the southern province are relatively aluminous, and plagioclase appears as a phenocryst phase before pyroxene. In the less aluminous northern province, pyroxene appears earlier than plagioclase. Within the northern province, three main fractionation stages may be distinguished on the basis of phenocryst assemblages, and may be used to classify the basalts.