38o H. C. COOKE 



accumulated under non-marine conditions, if Barrell's conclusion 

 as to the thickness of marine conglomerates is to be accepted. 

 The deposition of the conglomerate was followed by that of thick 

 beds of arkose and greywacke, locally with a little quartzite. A 

 limestone member may have been present in beds laid down beyond 

 the southern margin of the interior plateau. 



Following the deposition of the Mattagami series came a 

 period of intense orogenic movement and mountain-building, 

 during which all the rocks were folded closely along east-west 

 axes and gently along north-south axes. The more incompetent 

 rocks were converted into schists. Following the folding was the 

 great intrusion of later granite, which stoped away and digested 

 vast quantities of the older rocks, and which, now exposed by 

 erosion, underlies the major part of the region. 



A period of erosion followed, so long that the mountains of the 

 last folding were cut down nearly to base-level, and the granite 

 bathohths laid bare. No recognizable trace has yet been found 

 of the sediments which resulted from this erosion and which must 

 have been deposited somewhere during this period. The next 

 event of which any record remains is the deposition of the Bruce 

 series on the peneplained surface of the older rocks on the north 

 shore of Lake Huron. This indicates a third period of submer- 

 gence, which appears, however, to have been of small areal 

 extent. Emergence and the erosion of about i ,600 feet of the Bruce 

 series followed, with further gentle folding, after which a more 

 extensive submergence initiated the deposition of the Cobalt 

 series. At this time the sea transgressed certainly as far northward 

 as Lake Abitibi, and possibly to Lake Mistassini and beyond. 

 The nature of the Cobalt sediments indicates that the sea was 

 probably very shallow. Glacial conditions obtained at this time, 

 according to Coleman, M. E. Wilson, and others, and influenced 

 the character of the sediments laid down. The final event in the 

 history of the region, so far as traced here, was the intrusion of 

 gabbro and diabase into the older rocks in sills and dykes, preceding 

 or accompanying further gentle folding movements. 



Climate and life. — Our knowledge of the chmate and life condi- 

 tions that prevailed in these early times is exceedingly scanty, 



