400 E. M. BURWASH 



tation, as in the Rainy River region, "in others, sediments were 

 interbedded with the volcanics, and in other areas these early pe- 

 riods of sedimentation continued long after the extrusion of lavas 

 had ceased." In some cases a conglomerate exists which may mark 

 an unconformity between the two divisions of this pre-granite series, 

 as in the Lac Seul area; in others conglomerates occur at several 

 horizons in the upper sediments, but some of them are not of great 

 lateral extent, while graywackes and arkoses indicate little trans- 

 portation or sorting and possibly subaerial deposition. In some 

 places cross-bedded deposits are interpreted as ancient deltas. 



The whole would agree with the hypothesis that while post- 

 Keewatin mountain-building occurred in the Rainy Lake field and 

 others farther east, the country to the north and west, although 

 elevated above sea-level, remained in the condition of a piedmont 

 costal plain and, not being folded, was not intruded by granites of 

 Laurentian age.^ At the same time it was covered with the ill- 

 sorted products of mountain erosion, largely through the action 

 of torrential streams. At a later time, however, this region was 

 subjected to folding, batholithic intrusion, and subsequent deep 

 erosion. This was followed in some localities by one or more 

 periods of deposition with intervening folding and erosion, after 

 which long erosion had produced peneplanation before the ad- 

 vance of the Ordovician sea. 



This would agree very well with the facts observed by the writer, 

 except that the later sediments following the first folding (Upper 

 and Lower Missi, Churchill quartzites, etc.), do not occur in the 

 region on which this paper is based. 



^ An exception to this is made in the case of the Athapapuskow area in north- 

 western Manitoba which would appear to limit the area in which the Laurentian may 

 be absent on the northwest. 



