stripes
Alkaline rocks have been dredged from the Alphard Banks, which lie off the coast south and east of Cape Agulhas. The area is underlain by Mesozoic sediments within which are emplaced large numbers of small intrusions several of which protrude above the sea floor, to within 16 m of the surface, as a cluster of pinnacles. Rock types dredged are trachytes but with a little nepheline in some. They consist of phenocrysts of sanidine, aegirine-augite, occasional albite, a brown amphibole, titanite and in one rock altered biotite phenocrysts in a groundmass of sanidine and nepheline; in about half the rocks examined aegirine-augite, magnetite and sometimes glass occur. Two rock analyses are given by Dingle and Gentle (1972) one of which gives 23% normative nepheline. They suggest that the alkaline igneous rocks of the Alphard Banks be combined with the melilitite occurrences of Robertson (No. 151-00-042) and Spiegelsrivier (No. 151-00-043) as the 'Alphard Tertiary Igneous Province'.
DINGLE, R.V. 1970. Preliminary geological map of part of the eastern Agulhas Bank, South African continental margin. Proceedings of the Geological Society of London, 1663: 137-42.DINGLE, R.V. and GENTLE, R.I. 1972. Early Tertiary volcanic rocks on the Agulhas Bank, South African continental shelf. Geological Magazine, 109: 127-36.