ESTIMATES OF TIME 841 



The next assumption on the list is that the initial temperature gradient 

 of the earth at the time of complete solidification was a straight line with 

 a higher temperature at the surface than the fusion point of diabase, but 

 with a lesser slope, intersecting the diabase curve at some distance below 

 the surface. This primal temperature gradient is next assumed to have 

 been modified by two factors only — some new heat of radioactive origin 

 and a loss by conduction to the surface. Becker thus ignores, as others 

 had done, the evidence of the recurrence on a world-wide scale of the rise 

 of heat, manifested by batholiths and regional metamorphism — a convec- 

 tive overturn which would seem to have completely obscured the original 

 temperature conditions. 



By varying the initial surface temperature and the initial temperature 

 gradient he obtains six solutions for the age of the earth and the resulting 

 present gradient as dependent solely on cooling from the initial state. 

 The ages range from 30,000,000 to 100,000,000 years. He chooses the 

 60,000,000-year earth as being most probable, as indicated by other lines of 

 evidence. This yields a present thermal gradient from primal cooling 

 which amounts to 77 feet for one degree Fahrenheit. As this is not far 

 from the lower figures for the mean temperature gradient, Becker con- 

 cludes that radioactivity must be a minor factor in the heat of the crust. 



Thus, assumption is built on assumption into a many-storied structure 

 and the whole rests on a foundation of quicksand. That it is a castle in 

 the air and can not reflect tlie conditions of nature is indicated by various 

 well established inferences. For example, the deeper parts of many of 

 the older stratigraphic deposits, such as the Torridonian and Keweenawan 

 formations and the Precambrian series of the Grand Canyon of Arizona, 

 have been buried to a depth of several miles and are now exposed by 

 erosion. They are not metamorphosed and therefore show that ^ high 

 temperature gradient did not exist in the crust at that time except in 

 regions of metamorphism, where plutonic igneous invasion is often in- 

 dicated as the cause. Furthermore, measurements of the radium content 

 of the siliceous rocks, and tlie independence of its disintergration with 

 respect to pressure and heat, so far as laboratory experiments can ascer- 

 tain, show that to depths of at least some tens of miles radioactivity must 

 be a very important faictor in determining the present temperature gradi- 

 ent unless uranium and thorium should have tlieir disintegration arrested 

 by moderate pressure; but of this tliere is no evidence. 



Lane-^ has sliowii liow Becker could bave obtained difi^erent results by 



^' A, C. Lane : Schaerberle. Becker, and the cooling earth. Science, vol. xxvii, 1908, 

 pp. .589-592. 



