W. Upham — Estimates of Geologic Time. 211 



shed from the revolving molten earth, long before the forma- 

 tion 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 10,000,000 years are as much as phys- 

 ical science can allow to the geologist. Professor Newcomb, 

 summing -up the results of these physical and astronomical 

 researches, 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 18,000,000 



years at his present rate of radiation 10,000,000 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 cannot yet calculate with certainty, 

 but which cannot much exceed 20,000,000, 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 esti- 

 mate, 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 consti- 

 tution of the sun and earth. It must be gratefully acknowl- 

 edged, 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 

 of physics may be brought into harmony by revision of each. 



Among all the means afforded by geology for direct esti- 

 mates 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 

 thicknesses of the strata referable to the successive time 

 divisions. Now the rates at which rivers are lowering the 

 altitudes of their basins by the transportation of sediments to 

 the sea vary from an average of one foot taken from the land 

 surface of its hydrographic basin by the River Po in 730 

 years to one foot by the Danube in 6,800 years. As a mean 

 for all the rivers of the world, Alfred Russel Wallace assumes 

 that the erosion from all the land surface is one foot in 3,000 

 years. The sediments are laid down in the sea on an average 



* Popular Astronomy, pp. 5U5-519; Astronomy for Schools and Colleges, 

 p. 501. 



