and the Earth's Thermal History. 



105 



When such material is collected from the oldest rocks of the Archaean 

 shields, it is prohable that the ages will he found to approximate to 

 1500-1600 million years. 



For the purposes of this paper the period which has elapsed since 

 the crystallization of the oldest gneissose granites or orthogneisses, 

 a period which may he referred to as the age of the earth, will be 

 taken, independently of thermal considerations, as 1,600 million years. 

 "With such an extended period in which to cool, an earth originally 

 molten at or near the surface would now have a temperature gradient 

 so low- that it could easily he hidden under the much more imposing 

 gradient due to radio-thermal energy. "We must, therefore, examine 

 afresh the evidence for and against an earth originally molten at or 

 near the surface. 



_ , . ,_ . , Age in millions 



Geological Period. ' of years. 



Carboniferous, U.S. A 340 



Devonian, Norway ....... 370 



Silurian or Ordovician, U.S.A. . . . . . 430 



Pre-Cambrian : — 



Galle pegmatites, Ceylon 

 Unfoliated granites, G. E. Africa 

 Moss complex, Norway 

 Ser- Archaean, Sweden 

 Arendal complex, Norway 

 Algoman(?), Canada 

 Intrusive granites, Madagascar 

 Intrusive granites, Mozambique 

 Gneissose-granites, Mozambique 

 Thorianite, Ceylon . 



800 



800 

 1,000 

 1,100 

 1,250 

 1,200 

 1,250 

 1,100 

 1,500 

 1,600 (?) 



9. The Thermal Problem of the Original Crust. 



As already suggested, Chamberlin holds that the infalling of 

 planetesimals during the later stages of the earth's growth was so 

 comparatively slow that the heat generated was readily radiated 

 away. Indeed, he believes that oceans first became possible at 

 a stage when the earth was much smaller than now, and thus 

 denudation and deposition, both mechanical and chemical, began 

 while the earth was still growing by the capture of planetesimals. 

 Daly raises the objection ' that if this view is correet the ocean ought 

 to be more salt than it actually is. This criticism is undoubtedly 

 well founded, for the difficulty of reconciling the conflicting estimates 

 of geological time based on salinity and radio-active data respectively 

 becomes still worse if the ocean is considered to be older than the 

 oldest known rocks. Dr. Evans has suggested to me that if the 

 composition of the planetesimals which completed the growth of 

 the earth was at all similar to that of meteorites, then the earliest 

 known sediments, which must have been produced partly from fresh 

 planetesimal material and partly from volcanic rocks and pre-existing 

 sediments, ought to betray the peculiar nature of their source by 

 some peculiarity in their chemical composition. In particular, it is 

 reasonable to expect that they would contain determinable quantities 

 of nickel. This does not seeni to be the case, and in fact the earliest 



1 Igneous Rocks and their Origin, p. 115, 1914. 



