STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 377 



2. Leith^ has shown that the formation of the great Lake Supe- 

 rior syncHne began in pre-Keweenawan time, and suggests indi- 

 rectly that it may have begun to form very much earlier, even in 

 the pre-Huronian. The formation of a structure of this size must 

 have been accompanied by profound deformative effects on the 

 rocks for a long distance to the north and south. The evidence 

 from the parts of northern Ontario and Quebec dealt with in this 

 paper indicates that they have been affected by only one intense 

 folding movement of pre-Keweenawan age, the post-Mattagami 

 movement. The Bruce series, or Lower Huronian of the north 

 shore of Lake Huron, and the Cobalt series which overlies it, where 

 not affected by Keweenawan deformation are only gently folded 

 and rest on a folded and peneplaned surface. It seems reasonable 

 therefore to correlate the post-Mattagami folding with the primary 

 formation of the great synclines of Lake Superior and Hudson Bay. 

 The region between, under this hypothesis, is an anticlinorium, 

 and was probably mountainous, so far as can be judged from esti- 

 mates of the thicknesses of rock removed during the pre-Huronian 

 peneplanation. 



3. The Nastapoka sediments on the east shore of Hudson Bay 

 are classified by Leith^ as of Keweenawan age, with perhaps some 

 Animikee rocks included. The presence of these rocks shoWs that 

 at this time Hudson Bay was already a synclinal depression in 

 which sediments were being deposited. Their deformation shows, 

 in addition, that the folding movements by which it was formed 

 were still going on. This history is parallel to that of the Lake 

 Superior basin, the deformation of which continued throughout 

 pre-Cambrian time up to the end of the Keweenawan. 



REGIONAL HISTORY 



In the earliest times of which we have record the northern 

 Quebec region appears to have been a continental plateau of low 

 relief. This plateau included not only northern Quebec, Labrador, 

 and probably Hudson Bay, but also BafiEin Land, eastern Ontario, 

 parts of New York and Vermont, and perhaps a large area to the 



' U.S. Geol. Surv. Monograph 52 (19 11), p. 622. 

 ^Economic Geology, V (1910), 227-46. 



