Abstract
Summary: A mass of ijolitic rocks at Usaki, near Homa Bay in South Nyanza, Kenya, is described, together with comparable rocks from Homa Mountain and xenoliths from various parts of the Homa-Ruri area. Modal analyses are given of most of the rocks examined, together with chemical analyses of some of the main rock types and of some of their minerals. The origin of the rocks is discussed, and the suggestion made that their present constitution has developed largely by metasomatic replacement.