156 THE POPULAR SCIENCE MONTHLY. 



earth, long before the formation of its crnst. 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 

 Avhich 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 allot- 

 ments 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. Some- 

 where 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 care- 

 fully 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 fac- 

 tors 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 presiden- 

 tial 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 



