THE PRE-CAM BRIAN OF WESTERN PATRICIA 399 



biotite and brown iron oxide, very occasional grains of apatite and 

 magnetite and flecks of kaolin." The quartz pebbles in the con- 

 glomerate on the interprovincial boundary are of a rather more 

 glassy and massive appearance and must undoubtedly be described 

 as vein-quartz. They cannot be accounted for as coming from the 

 quartz veins which are now to be seen throughout the area, as these 

 are of later age and owe their origin to the subsequent granite 

 intrusions. But there is a bed of quartzite, as already described 

 in the Rice Lake rocks, which outcrops on the Winnipeg River a 

 little west of the Ontario-Manitoba boundary, and to this the 

 pebbles of the conglomerate may in part be attributed, while 

 others are no doubt derived from distant sources. 



Taking everything into account there seem to be fairly good 

 reasons for correlating the conglomerate horizons of the two areas 

 and also the volcanics which underlie them, which may be taken 

 as in all probability Keewatin. The conglomerate would then 

 agree in age with the great erosion interval which has been found 

 elsewhere after the Keewatin. In the region to the south and 

 east, however, the extrusion of the volcanics, partly at least under 

 water, was followed by mountain-building accompanied by granitic 

 intrusion and denudation. In the area which is now under review 

 there is no evidence of angular unconformity between the volcanics 

 and the sedimentaries. The sedimentaries may have been largely 

 subaerial in origin and in that case the history indicated is simply 

 (i) emergence, (2) erosion of low-lying land to a surface of low 

 relief, and (3) deposition of coarse materials perhaps on a pied- 

 mont plain. Some of these materials are of local origin while 

 others (the quartzes, the cherts in part, and the granites) have been 

 transported for greater distances. 



A recent paper by Drs. Alcock and Bruce^ sums up very well the 

 results obtained from a study of a number of areas in northern Mani- 

 toba together with that studied by Moore to the southeast of Lake 

 Winnipeg and the Star Lake area on the Ontario boundary farther 

 south — in all ten localities. Their general conclusion is that the his- 

 torical succession begins in most instances, as here, with very ancient 

 volcanic extrusions. This was preceded in some cases by sedimen- 



^ Bulk G.S.A., Vol. XXXII, pp. 267-92. 



