658 ALFRED W. G. WILSON 



areas of Paleozoic sediments suggests that the relatively rapid 

 depression which continued through Ordovician time continued 

 until near the close of the Silurian, and it is probable that the 

 middle part of the plain was completely submerged. The east- 

 ern and western parts of the plain were still above water-level. 

 If the middle portion was completely submerged at this time, it 

 must have been re-elevated and the sedimentary cover in part 

 removed, for Devonian corals have been found in the Hudson 

 Bay basin with their bases attached to bosses of Archaean rocks. 

 The Devonian sandstones flanking the peneplain on either side 

 also suggest that the middle portions of the plain were subject 

 to subaerial degradation during at least a portion of Devonian 

 time. Of the history of the greater portion of the region from 

 the close of the Devonian to the beginning of the Pleistocene 

 there is no sedimentary record closely associated with the pene- 

 plain. 



The late Mesozoic has been shown to have been a period of 

 extensive peneplanation throughout most of North America. 

 In New York and Pennsylvania to the south, in Wisconsin and 

 Michigan to the southwest, the remnants of the planation surface 

 have been recognized. In the far northwest strata classed as 

 Cretaceous are found resting apparently upon the peneplain 

 surface. The relations of Cretaceous sediments to a surface 

 apparently coincident with the plain cut upon the Archaean rocks 

 of Wisconsin also suggests that this plain is Cretaceous. It thus 

 seems not improbable that the planation processes of the same 

 period, working northward and eastward from these areas and 

 southward from the Arctic regions, may have in part produced 

 the younger of the two (or more) plains upon the Archaean 

 areas in Canada. 



The southern margin of the plain south of Methy portage and 

 around nearly to the city of Montreal is probably of Paleozoic 

 (and pre-Paleozoic) age, since there is little doubt but that it was 

 once covered with Paleozoic sediments. Whether this is true of 

 all that section which lies between the outliers before noted as 

 occurring in the basins of a number of lakes near the central 

 divide seems doubtful. The more detailed study of these basins 



