Abstract

Professor W. S. Pitcher said that the kyanite in the thin sections illustrated by the author had the appearance of being overprinted on all earlier structures: Dr Nicholson had also recorded a staurolite overprinting a crenulation cleavage, thus confirming the early observations of Combe. Could the author confirm such a late growth of kyanite and staurolite porphyroblasts, and how would this fit with his interpretations ? The speaker also asked whether, from his general knowledge of the country, the author could say to what extent the high-grade metamorphism was associated in space with the gneiss domes.

Dr R. B. McConnell said he was interested in the results of Dr Snelling’s age-determinations, which indicated that the gneisses in the centre of the dome were older than the surrounding sediments and that the structure thus resembled a mantled gneiss dome. When working in Uganda in 1950–51 the speaker was impressed by evidence that indicated that pre-Karagwe–Ankolean rocks had a greater extension than was generally accepted, and suggested that the gneissic cores of the domes in south-west Uganda, so well described by Combe, might represent modified pre-Karagwe–Ankolean rocks. In the Masha arena, for instance, 35 miles east-north-east of Ntungamo, although mantling Karagwe–Ankolean rocks had a generally concentric dome structure around the arena, the gneisses of the floor appeared to strike roughly ne–sw, which was near the commonest direction of pre-Karagwe–Ankolean rocks occurring to the east and north-east. Some support for this view was given by Phillips’s recent