374 tl. C. COOKE 



no limestone has been found, but on the other side of the Hne hme- 

 stone in considerable amount. 



The existence of such a definite boundary is of great significance 

 in considering the conditions of deposition of the Grenville series. 

 We have an interior region in which, for some reason, conditions 

 were unfavorable for the deposition of limestone; while, beyond the 

 margins of this region, limestone was deposited freely. Two pos- 

 sibilities present themselves to explain this fact. The portion of the 

 Grenville within the limestone boundary may be of terrestrial 

 origin, in which case the limestone boundary will represent approxi- 

 mately the location of the Grenville shore line ; or the whole series 

 may be marine, but the clastic sediment within the limestone bound- 

 ary may have been so abundant that limestone was not formed 

 there. In either case the limestone boundary represents a topo- 

 graphic break, where an interior plateau fell off sharply to a lower 

 level. The solution of this problem therefore depends on the 

 determination of the marine or non-marine origin of the Grenville 

 sediments within the limestone boundary. 



The data at hand are not sufficient for the solution of this prob- 

 lem, since the comparatively small Nemenjish area is as yet the 

 only one which has been studied. The known facts, pro and con, 

 may be summarized as follows: 



A. In favor of a non-marine origin of the series where limestone 

 is absent: 



1. The Nemenjish sediments as a rule have the composition of 

 an altered greywacke as if derived from older rocks by disintegra- 

 tion without much decomposition. They show small variations 

 in mineral composition, but these variations appear to be due to 

 variation in the composition of the older rocks from which they 

 were derived, rather than the variations due to normal weathering 

 and consequent separation into sandstone and shale constituents. 

 True quartzites, shales or slates, and limestones are conspicuous 

 by their absence, or their presence in very small amount, in the 

 northern region, although they are present in greater amount in 

 the marine division of the series beyond the limestone boundary — a 

 natural result of transportation over greater distances. 



2. Although remnants of the Grenville sediments appear to be 

 scattered over the whole of the Labrador plateau, it is not yet proved 



