earth, long before the formation of its crust. From the same arguments and the rate at which the sun is losing its store of heat, Prof. Guthrie Tait affirms that apparently ten million years are as much as physical science can allow to the geologist. Prof. Newcomb writes: "If the sun had, in the beginning, filled all space, the amount of heat generated by his contraction to his present volume would have been sufficient to last eighteen million years at his present rate of radiation. . . . Ten million years. . . is, therefore, near the extreme limit of time that we can suppose water to have existed on the earth in the fluid state." Not only the earth but even the whole solar system, according to Newcomb, "must have had a beginning within a certain number of years which we can not yet calculate with certainty, but which can not much exceed twenty million, and it must end."
The geologist demurs against these latter far too meager allotments of time for the wonderful, diversified, and surely vastly long history which he has patiently made out in his perusal of the volume of science disclosed by the rocks. He can apparently do very well with Lord Kelvin's original estimate, but must respectfully dissent from the less liberal opinions noted. Somewhere in the assumed premises which yield to mathematicians these narrow limits of time, there must be conditions which do not accord with the actual constitution of the sun and earth. It must be gratefully acknowledged, however, in the camp of the geologists, that we owe to these researches a beneficial check against the notion once prevalent that geologic time extends back practically without limit; and it is most becoming for us carefully to inquire how closely the apparently conflicting testimonies of geology and physics may be brought into harmony by revision of each.
Among all the means afforded by geology for direct estimates of the earth's duration, doubtless the most reliable is through comparing the present measured rate of denudation of continental areas with the aggregate of the greatest determined thickness of the strata referable to the successive time divisions. The factors of this method of estimate, however, are in considerable part uncertain, or dependent on the varying opinions of different geologists. According to Sir Archibald Geikie, in his presidential address a year ago before the British Association, the time thus required for the formation of all the stratified rocks of the earth's crust may range from a minimum of seventy-three million up to a maximum of six hundred and eighty million years. Prof. Samuel Haughton obtains in this way, "for the whole duration of geological time a minimum of two hundred million years." On the other hand, smaller results are reached through the same method by Dana, who conjectures that the earth's age may be
