THE LAURENTIAN PENEPLAIN 655 



The forests extend over the western portion of the plain to a 

 short distance north of Great Slave Lake. Beyond this the 

 region is treeless, moss-covered tundra. South of the forest line 

 there are frequently large areas floored with glacial drift and 

 forming numerous muskegs (or swamps) above whose surfaces 

 the crystalline ridges and glacial kames and eskers project. 



SOME PROBLEMS OF GENERAL INTEREST. 



Aside from the strictly geologic problem of the origin and 

 relations of the rock complex of which the Canadian shield is 

 composed, there are a number of secondary problems of interest 

 alike to the geologist and to the physiographer. Although it is 

 not proposed to enter into an elaborate discussion of any of these 

 problems at present, the time seems not inopportune to outline 

 a few of those which are of general interest, and which are closely 

 associated with one another. 



I. The for7ner extent of the Paleozoic cover. — The first of these 

 which suggests itself is one to which reference has often been 

 made. Did the Paleozoic sediments ever extend across the 

 interior divide? Opinion seems somewhat divided on the answer 

 to be given to this question. No doubt additional light will be 

 thrown upon it when the fauna of the Paleozoic sediments occur- 

 ring to the south of Hudson Bay can be adequately studied. 

 The occurrence of the inlying areas of Paleozoic deposits, already 

 referred to as occurring in the several depressions between Lake 

 Mistassinni and Lake Nipigon, is generally regarded as strong 

 evidence of the extension of these sediments at least across that 

 portion of the plain which lies to the south of Hudson Bay. 



To the physiographer the problem has an added interest from 

 the fact that, if these sediments once extended completely across 

 the peneplain, the questions arise: Is the present surface of that 

 plain approximately the one upon which they rested ; or has the 

 surface upon which they were deposited been removed long 

 since? Would it be represented by the extension of the pene- 

 plain of earlier date (which, it has seen shown above, probably 

 exists) toward the locus of the present main divide? 



The answer to these queries seems to be intimately associated 



