STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 201 



fact that the conglomerate member is found only on the north side 

 of the band of sediments. It seems more probable that they form 

 the north limb of a syncline, whose south limb has been stoped off 

 by the granite, while its anticlinal continuation to the north has 

 been removed by erosion. If this is the case, the axial plane of this 

 syncline must dip to the north. 



Both Wilson and Bancroft have described all the sediments as 

 parts of one sedimentary series. Since their work was published, 

 however, several facts have come to light which are difficult of 

 explanation if the rocks are to be considered as a single series, while 

 the fact that outcrops are exceedingly poor in this district, owing 

 to the heavy cover of glacial lake clays, renders it possible that two 

 or more series of metamorphosed sediments have been inadver- 

 tently grouped together. The facts which tend to throw doubt on 

 their interpretations are: 



1. On Lake de Montigny, at the contact of the Pontiac series 

 and the underlying greenstones, no conglomerate is found. The 

 basal beds of the sedimentaries are dark mud rocks of the compo- 

 sition of a basic greywacke. Conglomerate does not occur here. 

 Wilson describes the conglomerate as occurring in discontinuous 

 patches. In the series of younger sediments to the north, already 

 described by the writer, the conglomerate band at the base of the 

 younger sedimentary series is always thick, strong, and con- 

 tinuous. 



2. Wilson describes amphibolites occurring interbedded with the 

 mica schists of the Pontiac series in places. Amphibolites were not 

 observed by the writer to form part of any of the younger sedimen- 

 tary series in any of the areas studied by him; they are, however, 

 very characteristic of the Nemenjish series and form a considerable 

 proportion of it. 



3. Wilson"" found outliers of the Grenville series (which, it will 

 be shown, is probably correlative with the Nemenjish series) on 

 Grand Lake Victoria, about twenty-five miles south of the most 

 eastern outcrop of the Pontiac series. Bancroft^ in 19 16 found out- 

 liers of the Grenville series as far west as Doucet, some thirty miles 



^ Geol. Surv. Can., Sum. Rept., 1912, p. 337. 

 ^ Personal communication 



