THE EARLY PRE-CAMBRIAN FORMATIONS OF 



NORTHERN ONTARIO AND NORTHERN 



MANITOBA 



E. L. BRUCE 



Queen's University, Kingston, Ontario, Canada 



The classification of rocks earlier than the Cambrian is one of 

 the difficult problems of geology. Correlation from point to point 

 rests largely upon lithology and upon the succession of lithological 

 units, and difficulties arise both from original similarity of forma- 

 tions of quite different age, and from similarity through the develop- 

 ments of the same metamorphic minerals in rocks originally of 

 quite different character. Correlation is especially difficult in 

 formations of early pre-Cambrian time, since these have undergone 

 much longer periods of deformation than have the later ones. 

 Varying successions of these early rocks have been worked out in 

 some detail in different parts of the Canadian shield, and many 

 attempts have been made to formulate a generalized succession 

 that will fit all areas. None of these attempts has been successful, 

 since the determination of the age of a formation by its likeness 

 to a certain formation as described in an accepted classification, 

 is Hable to lead to quite erroneous conclusions. A review of some 

 areas in western Ontario and northern Manitoba will show the 

 diversity in various sections and will, it is beheved, make possible 

 certain generalizations. 



RAINY LAKE DISTRICT 



The first attempt to subdivide the early complex was made 

 by Lawson^ in the Rainy River district. He recognized two 

 formations earlier than the first granite intrusion. The lower of 

 these is his Coutchiching series, which he believes to be the oldest 

 formation in the area. The Coutchiching rocks consist of mica 



I Geol. Survey of Canada, Ann. Rept., New Series, Vol. Ill, Part I (1887-88). 



459 



