26 FRANK D. ADAMS 



found in the pre-Cambrian in sufficient abundance to justify a zoic classification, 

 there can be no sufficient warrant for proposing that the major divisions of the 

 pre-Cambrian be made upon a zoic basis. 



CLOSING DISCUSSION BY THE AUTHOR 



The aim of the paper on "The Basis of Pre-Cambrian Correlation" was, as 

 stated, to suggest a method by which it might be found possible to correlate the 

 various subdivisions of the pre-Cambrian rocks over widely extended areas rather 

 than to enter upon a discussion of the classification of the pre-Cambrian of North 

 America. 



With regard to this latter classification, however, it must be pointed out that 

 the paper shows that in a general way the classification adopted by the Inter- 

 national Committees (United States and Canada) on the "Correlation of the Pre- 

 Cambrian Rocks of the Lake Superior Region" and on the "Pre-Cambrian Rocks 

 of the Adirondack Mountains, the Original Laurentian Area of Canada and 

 Eastern Ontario," probably forms a satisfactory basis upon which the classifica- 

 tion of the whole expanse of the great pre-Cambrian development of the Lauren- 

 tian protaxis can be founded. Professor Van Hise is mistaken in stating that in 

 the paper under discussion the succession recognized by these committees was 

 adopted but that the unconformities were omitted, for in the wall diagram used 

 to illustrate the paper, and upon which the succession of the pre-Cambrian 

 rocks in Laurentia and China was set forth, the unconformities were especially 

 indicated, black lines being used to show those which were of minor importance 

 while broad red lines appropriately emphasized the major breaks in the succes- 

 sion. The unconformities and their relative importance are also shown in the text 

 of the paper. In fact, this is the crucial point of the paper so far as Laurentia is 

 concerned. 



Professor Van Hise has insisted, in a long series of papers, that in the pre- 

 Cambrian succession of North America there is one break which in importance far 

 transcends all others, namely, that at the close of the Keewatin. Professor Law- 

 son, however, has insisted that in this succession the chief break lies at quite a 

 different horizon, namely, at the base of the Animikie. 



The International Committees, while recognizing the succession of the various 

 elements of the pre-Cambrian, absolutely declined to commit themselves to any 

 opinion as to the relative magnitude or importance of the several unconformities 

 which they recognized. 



A study of all the work — much of it recent — which has been done in the more 

 northern portion of Canada indicates that Professor Lawson's break — the Epar- 

 chaean Interval as he terms it — is one of the greatest unconformities in the whole 

 pre-Cambrian succession of Laurentia, and probably quite as important, if not 

 more so, than the break at the close of the Keewatin, and that the pre-Cambrian 

 rocks are represented, not by two great systems entirely distinct and separated 

 from one another, but by three great systems. 



