Even during the progress of a campaign, when complete information is necessarily unattainable, it is well to assemble facts and probabilities in an orderly manner, and to make a review of the situation as a guide to further action. It is as yet far too early to pronounce finally upon the part which radioactivity will eventually play in cosmogonic theories, but not too early to consider the bearing of recent discoveries on the genesis and past history of the globe. Since geologists to whom this paper is addressed can not be expected to be very familiar with radioactivity, I shall first make an attempt to sketch in outline such features of that subject as seem to me of especial interest to us; next I shall try to show under what conditions radioactive substances can come into existence, and finally discuss the geological effects which may be expected from them. . . .