II.—Radio-activity and the Earth's Thermal History

Two years ago, in discussing the thermal energy of the earth, I suggested that while it had become impossible to deduce the earth's age from its thermal condition alone, Kelvin's problem might profitably be reversed by accepting the earth's age as a known factor, and deducing with its help the thermal history of the earth. This paper is a first attempt to attack the new problem then suggested. For geological purposes, one of the most fundamental aspects of the problem is that relating to the depth within the earth's crust at which temperatures are attained such that, under suitable conditions of pressure, molten rock magmas may exist. The determination of the minimum depth of possible rock fusion is a first essential to any adequate theory of vulcanism, and indeed of igneous activity in general. It is not sufficient, however, to ascertain that depth for present conditions alone, its variation during the earth's geological history must also be investigated ; for if, as is generally believed, the earth is a cooling body, the depth must be slowly increasing, and in former periods it must necessarily have been nearer the surface than it is now. In the limiting conditions both of position and time, tlie depth of fusion may have been at, or so near as to be for all practical purposes at, the surface itself. That is to say, at the beginning of geological history the earth may have been in a molten condition at, or immediately below, the then existing surface