STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 273 



that as might be expected, the extrusions were more or less local- 

 ized, so that the Nemenjish series may lie from place to place on 

 different types, but still maintain conformable relations. Further, 

 it appears that the sedimentation was slight during the whole 

 extrusive period, and largely confined to deposits directly or 

 indirectly from volcanic sources. Lava flows may be interbanded 

 with these sediments, but the extrusion of these was complete 

 before the true elastics became large in amount, as the two have 

 never been found interbanded. 



Relations to younger formations. — Since the Nemenjish series 

 rests conformably on the lavas in the Windy Lake area, and the 

 Mattagami series rests unconformably on the lavas in the Brock 

 and other areas, it might be supposed that the Mattagami series 

 is younger than and unconformable on the Nemenjish series. 

 This conclusion assumes that the lavas in the different areas are 

 of the same age. Direct field evidence bearing upon the relation 

 of the two sedimentary series is very scanty. The only areas 

 furnishing such evidence are the Kenoniska and Lucky Strike 

 areas, where garnetiferous mica schists are found lying between 

 the Mattagami basal conglomerate and the underlying lavas. 

 The petrography of these rocks makes their correlation with the 

 Nemenjish seem probable, and they lie unconformably beneath 

 the Mattagami series. 



The Nemenjish-Grenville series is intruded by all the plutonic 

 rocks of the region, and is therefore older than all. An older 

 granite intrudes it in the vicinity of Lake St. John. Later, it 

 was cut by great masses of anorthosite. Finally, it was cut and, 

 in this region, almost entirely stopped away and digested by the 

 great batholithic intrusions of granite which followed the deposition 

 of the Mattagami series. 



LAVAS 



The lavas of the region under discussion appear to be of a 

 single age, so far as the evidence at hand indicates. They are 

 very similar in their petrographic character and in the extent 

 of their metamorphism. Wherever their succession has been 

 worked out, it is the same; basaltic lavas form the lowest exposed 

 beds, overlain successively by andesitic lavas, andesitic tuffs, and 



