STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 371 



identical with the rocks of the Grenville series are found up to the 

 shores of James Bay. 



Alternative to the hypothesis of marine transgression lies the 

 possibility that at this time the land was covered by a series of 

 large lakes. If the land remained above the level of the sea, the 

 outpourings of lava that took place must have filled the stream 

 vallej's and utterly disorganized the drainage; lakes must have 

 been formed as a result, and if the region was one of low relief, as 

 it will .presently be shown appears probable, these lakes may have 

 attained sizes comparable to those of the great glacial lakes of 

 Pleistocene times. Such a condition would explain the facts stated, 

 as well as marine transgression. 



Topographically, the surface on which the lavas were poured out 

 seems to have been one of low relief. Had high mountains existed 

 at the time, the remains of the peaks, around which the lavas would 

 have flowed, should surely be found now penetrating into at least 

 the lower beds above, which are now on edge. Nothing of the sort 

 has come to light. Again, had the extrusions -been thrown out from 

 high peaks, such as the volcanic cones of the present day, we might 

 now expect to find sudden changes of strike near the volcanic centers, 

 due to the occurrence of steep depositional dips. This has not been 

 observed. The conclusion may therefore tentatively be drawn that 

 the flows were extruded as fissure eruptions, at least in the case of 

 the more widely distributed basic lavas, and to have flowed out over 

 a surface of fairly low relief. More detailed field work is necessary, 

 with careful mapping of the areal boundaries of separate flows, to 

 establish this conclusion. 



Conditions of Deposition of the Grenville Series 



Areal distribution. — -A careful search has been made of the 

 reports of the explorers of northern Quebec and Labrador, and 

 determinations made from their descriptions of the location of 

 areas of the Grenville series, using as criteria for its determination 

 mainly petrographic similarity and geological relations. Many 

 descriptions were met with of rocks which bear a strong resemblance 

 to certain phases of the Grenville, but are likewise indistinguishable 

 from the Pontiac or some other highly altered sedimentary series. 



