ORIGIN AND DIFFERENTIATION 551 



fact that there is very little Cambrian, no Ordovician, Silurian, or 

 Devonian, and comparatively little Pennsylvanian in this section. 



While the above may be a satisfactory explanation of the specific case 

 cited, it may not explain lack of regional metamorphism in general. 

 This point is touched on by Manson ^* in his recently published paper, 

 his statement reading as follows : 



"The conductivity of the crust of the earth is so low that the heat which 

 could be made available as a climatic factor through this process [liberation 

 of earth heatl is negligible, except for very short periods over very limited 

 areas. This extremely low conductivity was one of the highly conservative 

 factors imposing the long duration of this source and its liberation is recorded 

 in the altered and unaltered sedimentations and severe crustal ruptures up to 

 the present era." 



This interpretation means that earth heat, given off very slowly owing 

 to the low conductivity of the rock crust, was stored in warm oceans. 

 The warm waters increased evaporation and cloud formation, thus con- 

 serving the heat, which process was also aided by the sun's rays falling 

 on the outside of the more perfect cloud spheroid. This slow trans- 

 mission of heat through the rock crust has not only acted as a conservator 

 of earth heat, but explains why rocks brought from great depths show 

 so little evidence of having been subjected to great heat. 



His third contention is that the "quantity of heat which the eartli 

 delivers to the atmosphere is now, and must always have been, inconse- 

 quential in comparison with that derived at present from the sun" ; 

 hence, for example, ^'Ho have conducted five times the heat, the tempera- 

 ture gradient would have to be five times as steep, giving a temperature 

 of molten rocks at a depth of five miles.'' 



This ignores wholly the value of radioactivity, and in another part 

 of the same paper (p. 839) Barrell says: "The discovery of radioactivity 

 cuts out all solid basis for calculating age. [of the earth] from the flow 

 of solar energy or the temperature gradient of the earth." N'o furthei' 

 comment on this point seems necessary. 



Barren's fourth proposition is as follows : 



"The presence of banding in certain argellites of early pre-Oambrian times 

 in Norway and Canada, associated with ancient glacial deposits, is directly 

 comparable with the annual banding in stratification in Pleistocene clays in 

 those same regions, and testifies to the dependence of temperature in those 

 times on solar radiation with an atmospheric condition which permitted the 

 existence of winter." 



3* Marsden Manson: Geologic and present climates, 1919, p. 217. 

 XXXVIII— Bull. Gkol. Soc. Am., Vol. 30, 1918 



