Ten years ago I published two papers in this Magazine under the above title. They constituted a first attempt to attack the new problems suggested by the universal presence of the radioactive elements in rocks, the discovery of which by the present Lord Rayleigh made it immediately necessary to abandon Kelvin's classical theory of the cooling of the earth. In 1913 I had already pointed out the possibility that Kelvin's problem might profitably be reversed, and that instead of calculating the age of the earth from its thermal condition, the latter, that is to say the distribution of temperature in depth, might be deduced from the earth's age. Two years later the new problems were brought to a tentative solution on the general assumption that the earth had cooled down from a molten state, an assumption which, although traditional, was at that time less clearly justified than it is to-day. As a working hypothesis it was further assumed that the total amount of the radioactive elements in the earth was limited by the condition that the earth had in fact cooled down and was not, as seemed to-be a possibility, growing hotter. Taking the age of the earth as Holmesthe Rockies (60 kins.), have recently been considerably increased. For these reasons geologists have rejected the new thermal contraction theory as being by itself still inadequate. My own opinion was that the theory explained about a third or a quarter of all the folding and overthrusting that has demonstrably occurred during the earth's known history.