In an article by Dr. H. Jeffreys in the Geological Magazine for April, pp. 215–19, an attempt is made to show that the discovery of radio-active materials in the earth's crust favours the “contraction and puckering” theory of mountain building. With regard to 0. Fisher's view he writes, “It rests entirely on Kelvin's theory of the cooling of the earth, which has had to be completely revised on account of the discovery of the extensive distribution of radio-active matter in the earth's crust. The time available has been found to be about twenty times greater than on Kelvin's theory, and the cooling has therefore had time to extend to a much greater depth and to produce a very much greater compression.” We are also told what the reduction in the earth's diameter has probably been, and the actual depth of Fisher's level of no strain.