Ngurumanite, a New Hypabyssal Alkaline Rock from Kenya

IN June 1959, E. P. Saggerson and B. H. Baker of the Geological Survey of Kenya discovered a number of small alkaline intrusions in upper Tertiary lavas on the Nguruman Escarpment, which forms the western wall of the Rift Valley about seventy miles south west of Nairobi. During a detailed examination of these intrusions in 1960, we found the alkaline rocks to be emplaced in a flow of ankaratrite (biotite-olivine melanephelinite), that is evidently intercalated in more common olivine basalts. The intrusions, which are probably of upper Tertiary age, are closely controlled by two north–south vertical fault zones in the ankaratrite. Narrow dykes feed sills about 2 ft. thick and these thin rapidly into an intricate network of veins.