We shall now consider the evidence which geology seems to afford as to the age of the sun’s heat. Geology is quite competent to render aid on this point, for the sun’s heat must be at least as old as life on this globe; and the record of the rocks tells us when this life first appeared. We require, however, to be able to measure the time which has elapsed since these records were left. What we want is absolute time; not relative time. Much has been done by geologists in regard to relative time; but this can be of no service to us in our present inquiry. Unfortunately very little trustworthy work has been done in the way of determining the absolute length of geological periods. Happily, however, great exactness of measure is not required. A rough approximation to the truth will suffice for our present purpose. If it can be shown to be more than fifteen or twenty millions of years since life first appeared on the earth, it will as effectually prove that gravitation alone could not have been the source from which the sun derived his heat as if it were shown that that period was a thousand times more remote. All we have to do is simply to assign an _inferior limit_ to the age of life on the earth; and this can be effectually done by means of the methods, imperfect though they be, which we have at command. As the question of geological time is of some importance in relation to our present inquiry, I shall consider it at some length. _Testimony of Geology: method employed._—What has subsequently proved to be a pretty successful method of measuring geological time suggested itself to my mind during the summer of 1865. It then occurred to me that we might obtain a tolerably accurate measure of absolute geological time from the present rate of subaërial denudation, which might be ascertained in the following way: The rate of subaërial denudation must be equal to the rate at which materials are carried off the land into the sea; and this is measured by the rate at which sediment is carried down by our river systems. _Consequently, in order to determine the present rate of subaërial denudation, we have only to ascertain the quantity of sediment annually carried down by the river systems._ This gives us the time required to remove any given quantity, say one foot, off the face of the country. If we assume the rate to be pretty much the same during past geological ages, we have a means of telling the time that was occupied in removing any known thickness of strata. But as we never can be perfectly certain that the rate is the same in both cases, the results can, of course, be regarded as only approximately true.