Similar uncertainties beset us when we consider the rate of chemical denudation, that is, the rate at which salts have been dissolved from the lands and accumulated in the oceans, as a measure of geological time. Here, again, we take the total amount of salts that is in the oceans today and divide it by the present rate of annual supply. We know with reasonable accuracy the quantity of salts in the oceans and if it were possible to assume the present rate of supply to be a true mean for all geological time, a satisfactory age of the oceans might be obtained. But it cannot be assumed as such. An assumption of this nature will only lead us from the domain of exactness to that of uncertainty. Aside from various other factors, neither the area of the continents, nor their relief was in the past the same as today. Consequently, the stream gradient and its power of dissolving salts from the land surfaces have not been the same. It is also not known how much salt the ocean derived from the shore line and from beds beneath the ocean, nor how much of the rock-salt beds on the earth that has been precipitated out of ocean water. It is plain, therefore, that the rate of any process that is controlled by so many conditions cannot be used (even making generous allowances for irregularities and inaccessible data) as a reliable guide to evaluate geologic time. “It is a clock,” says Harker, “which now hurries and now creeps or stands still, and it cannot be trusted as a timekeeper.” Any estimate based on the temperature of the earth, or of the sun, encounters similar practical difficulties, for the temperature of a body may not be constant. It may rise or it may fall. Further, the rate of change of temperature is controlled by a variety of conditions, such as the amount of energy radiated, the supply of energy and so forth. Nor is there any record of the immense quantity of heat produced by igneous agencies and radio-activity. Another estimate, one of the earliest, was based on the rate of life transformation in successive periods. The geological series were divided into twelve periods and it was believed that 20,000,000 years were required for an entire change in the species of each period, or 240,000,000 years in all. This does not include the time in which we have no record of plant or animal life.