Professor A. P. Coleman — Radio-activity , etc. 273 



V. — " RaDIO-ACTIVITY. AND THE Earth's THERMAL HlSTORY " '. 



A Criticism. 

 By Professor A. P. Coleman, M.A., Ph.D., F.E.S., University of Toronto. 



IN the very interesting paper by Mr. Arthur Holmes on " Radio- 

 activity and the Earth's Thermal History ", in the Geological 

 Magazine for February and March, it is assumed that in Archaean times 

 there was "a widespread molten condition at no very great depth" 

 below the earth's surface, and in the final conclusions the statement 

 is made that " geological and other evidence points favourably to the 

 traditional view that the earth's crust was initially in a molten state ". 

 The results of recent work on the Archaean of Canada do not bear 

 out this traditional view of the conditions of the earth's earliest known 

 geological period, and it may not be amiss to suggest some points that 

 decidedly conflict with it. 



The earliest rocks known on the Canadian Shield are the 

 Coutchiching, the Keewatin, and the Grenville Series. The 

 Coutchiching consists entirely of metamorphosed muddy and sandy 

 sediments, the Keewatin includes mainly volcanics, but also con- 

 siderable amounts of sediments, and the Grenville is essentially 

 sedimentary and consists largely of limestone. These three groups 

 of rocks seem to have been formed in succession with no great 

 break in time. Lawson has shown that the Coutchiching, in part 

 at least, underlies the Keewatin, and Miller and Knight have found 

 that in Eastern Ontario the Grenville overlies Keewatin volcanics, 

 apparently conformably. All of them antedate the batholithic 

 eruptives, which have upheaved and penetrated them. They were 

 therefore deposited before the earliest known acid plutonics, generally 

 called Laurentian, had begun their ascent beneath the Archaean 

 mountain ranges. 



Lawson gives the Coutchiching sediments of Rainy Lake a thickness 

 of 4-g- miles, and the Keewatin volcanics he describes as equally thick. 

 In other Keewatin regions 1,000 or more feet of " iron formation " 

 as well as important amounts of other sediments are found associated 

 with the Keewatin and should be added to the total. The Grenville 

 of the Haliburton region in Eastern Ontario is believed by Adams to 

 be 90,000 feet in thickness, the greater part consisting of limestones. 



It will be seen, then, that before the first known batholithic 

 mountain building began there were on the surface of the earth many 

 thousands of feet of sedimentary and volcanic rocks. It may be 

 objected that the thickness of these ancient and much metamorphosed 

 rocks has perhaps been over-estimated, but it must be remembered on 

 the other hand that they are known to us only as remnants, since the 

 great mountain ranges of the early Archaean had been reduced to 

 ■a peneplain before the beginning of the Huronian. It is most 

 unlikely that the whole thickness of these rocks has been anywhere 

 preserved. 



But there is another point of equal importance to be considered. 

 The floor on which these great beds of sediments and volcanics were 

 deposited has nowhere been found, though it must have been thick 

 and solid to support their weight ; and the great areas of dry land 



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