PEE-CAMBKIAN. 69 



The first interpretation (Laurentian in general considered as the equivalent of 

 undifferentiated pre-Cambrian), was found to be unacceptable to the Canadian repre- 

 sentatives. Prof. Adams had earlier in the discussion expressed the opinion that 

 the Laurentian in the sense of pre-Huronian constituted essentially 99 per cent of 

 the northern part of the Canadian shield. 



In answer to a question by Mr. Willis as to whether the Canadians were prepared 

 to restrict Laurentian to the equivalent of pre-Huronian, Dr. Brock -responded: 



I tliink we were all agreed at Ottawa that this would be the most desirable thing to do if 

 possible, but we overruled it on account of the difficulty in separating the Keewatia and Huron- 

 ian. It was necessary to have some color which would indicate Keewatin and Lower Huronian 

 for purposes of mapping. Economically they go together. If we could possibly do it we would 

 very much prefer to have a general color to cover the lower part, but in mapping I do not think 

 we can do it. We have many areas in which we can not differentiate the Keewatia and Lower 

 Huronian. 



Prof. Chamberlin asked: 



What is your preference, Mr. Brock, in regard to the lower formation ? Should it represent 

 undifferentiated pre-Cambrian or should it represent Archean ? 



Dr. Brock responded: 



We would prefer to see it represent the Archean. There is, of course, undifferentiated 

 Huronian in that probably, but the main fact is that the great part of the northern area repre- 

 sents what Logan called Laurentian. 



Prof. Chamberlin urged the recognition of Logan's definition of Laurentian. 

 He said : 



From my point of view I should like to see the general color [adopted for the Canadian 

 shield] represent pretty closely Logan's idea, which involves Keewatin, Laurentian intrusive 

 into Keewatin, and rocks of general Laurentian aspect — the old gneiss groups as confessedly an 

 undifferentiated series but as a great series representing one of the greatest of the eras of the 

 earth's history; so that if this conception is true we have the great nucleus of the continent. 

 I am thinking of something more fundamental than pre-Cambrian, something nucleal to 

 Huronian, Animikie, and Keweenawan. We need to find the general nucleus from which the 

 Canadian formations have been derived. 



Prof. Van Hise granted the fact that the nucleus described had existed for 

 Paleozoic formations but did not admit that the Canadian region was a nucleus 

 for the pre-Cambrian. 



As a result of further discussion the term Laurentian in general as applied to 

 the unclassified pre-Cambrian was stricken out. The use of Laurentian in general 

 as equivalent to Archean in the restricted sense of pre-Huronian was recognized 

 as one having a theoretical significance, but one which in the present state of knowl- 

 edge does not represent a mappable thing. There remain the two usages, namely, 

 (1) Laurentian equivalent to the Keewatin and the intrusive rocks cutting the 

 Keewatin and including probably some Huronian " and (2) the Laurentian intru- 



a In commenting on these minutea of the conference, Prof. Adams, under date of March 9, 1908, wrote: "In this 

 usage the Laurentian includes not only rocks which are equivalent to Keewatin but any portion of the pretaxis which 

 may be found to be older than Keewatin; that is to say, if in the great northern protasis the base exists on which the 

 Keewatin has been deposited, this also is of Laurentian age. It may easily be that a not inconsiderable proportion 

 of the Laurentian protaxis is really pre-Keewatin in date." 



