STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 375 



that they once formed a continuous sheet over this area. It is 

 equally possible that they may have been deposited in discontinuous 

 basins. 



3. If the origin of the sediments was marine, a study of the map 

 shows that it will be necessary to postulate that the land area from 

 which they were derived occupied part of the present area of Hudson 

 Bay. It is difficult to conceive of the transport of sandy sediments 

 from a land area in this position to the eastern Ontario district 

 or the Labrador coast. 



B. In favor of a marine origin: 



1. The entire lack of conglomerates or coarse-grained clastic 

 sediment in the known portions of this series is strongly against 

 the hypothesis of a non-marine origin. Deformation has not been 

 so great as to destroy such textures, as their preservation in the 

 Mattagami series shows. 



2. In the Nemenjish area all the rocks are well bedded, usually 

 in beds twenty or more feet in thickness. No trace remains of 

 cross-bedding, lens-shaped beds, or other structures characteristic 

 of terrestrial deposits. This area might of course have been part 

 of the site of a large lake. 



3. The greywacke-like composition of the rocks might equally 

 well be explained on the basis of the lack of vegetation in these early 

 times, so that the composition might not argue either for or against 

 the marine origin. Where vegetation was lacking and precipitation 

 abundant, sediment would be removed as fast as disintegration 

 took place, so that complete decomposition of the rock constituents 

 by weathering might not have taken place. 



Conditions of Deposition of the Mattagami Series 



Areal distribution. — The Mattagami series is not known certainly 

 outside of the region under discussion, as its existence and age 

 relations have been recognized only within the last few years. 

 Apparently, however, it was once fairly uniformly distributed over 

 northwestern Quebec, at least as far north as the East Main River, 

 where sediments of similar composition and degree of deformation 

 are described by Low.^ The writer has not been able to identify the 



' Geol. Surv. Can., Ann. Rept., VIII (1896), Part L. 



