It may be taken for granted that everybody agrees with Dr. Molengraaff when, in discussing Dr. Hatch's paper on “The Oldest Sedimentary Rocks of the Transvaal,” he says: “There must exist, or have existed, a formation older than the Witwatersrand Beds, because even at the very base of the Hospital Hill series conglomerates are known, the pebbles of which cannot obviously have been derived from the Hospital Hill series by denudation.” It has, therefore, I take it, not been the intention of various authors to prove this self-evident truth, but only to try and definitely settle the still open question whether this older rock is yet represented in the geological sequence of the country, and, if so, what strata should be identified as such. The relative age of the ‘old granite’ or ‘grey granite’ has been the base and the cornerstone of controversy on this subject, and it therefore becomes imperative to put on record and to continually keep in mind the only reliable mode of determination of the age of eruptive rocks which we yet possess. It would be out of place here to insist upon the origin of abyssal eruptive rocks of the granite family (Rosenbusch's Tiefengesteine).