Or the Norian or "Upper Laurentian" Formation. 435 



margin of the main Archaean protaxis of the North 

 American continent, exactly as the volcanoes of the pre- 

 sent time lie along the edges of continents. 



7. They are undoubtedly of pre- Cambrian age, and 

 have probably originated toward the close of the Lau- 

 rentian. 



8. The Laurentian system in the eastern part of Cen- 

 tral Canada consist of two sub-divisions, which were by 

 Logan classed as Lower Laurentian : 



(1) Tapper, or Grenville division ; 



(2) Lower, Ottawa or Fundamental gneiss. 



The Grenville division contains crystalline limestones, 

 (juartzites, and various kinds of gneiss, which are mostly 

 distinctly foliated, banded, or stratified, dipping very 

 often at a low angle and extending over large areas, the 

 rocks being in many places rich in finely disseminated 

 graphite, beds of iron ores, etc. 



The Lower, or Ottawa, gneiss alone is present in the 

 western part of Central Canada, where Lawson made his 

 well-known investigations on the relations of the Huronian 

 to the Laurentian gneiss. 



The lower and the upper divisions are so closely related 

 to one another that it is generally difficult to determine 

 accurately their geographical limits. It is possible that 

 they may form a continuous series laid down under con- 

 ditions which approached more and more closely to those 

 of modern times, or that the Grenville division lies uncon- 

 formably on the older gneisses and represents quite another 

 set of conditions from those under which the lower series 

 originated. 



The latter view is probably the correct one. 



9. The Canadian anorthosites resemble exactly certain 



other anorthosites which are found associated with Archaean 



rocks in Norway, Kussia, and in Egypt. The Norwegian 



occurrences are probably more recent in age than those 



found in Canada. 



32 



