Summary: IN 1931, H. V. Ellsworth analysed a specimen of uraninite from Manitoba and obtained a lead-ratio with the surprisingly high value of 0-260, correspond ‘ ing to an age of about 1,750 million years. Although Ellsworth gave adequate evidence that the mineral was of first class quality, there has naturally been some hesitation in accepting this great extension of geological time. Confirmation of the most convincing kind is now, however, forthcoming. It is announced by Prof. A. C. Lane through Science Service, Washington, D.C., that Miss Edith Kroupa (working in the laboratory of Dr. F. Hecht in Vienna) has analysed a sample of monazite which occurred with the Manitoba uraninite. The age turns out to be 1,725 million years. The significance of this high figure may be realised when it is remembered that the ‘Middle’ Pre-Cambrian rocks of Ontario, Norway and India have an age of about 950-1,050 million years. Clearly there was time enough before this for at least three major cycles of mountain-building and igneous intrusion, of which only one has hitherto been generally recognised.