STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 75 



mau Mining Commission. Because of its undeformed character 

 and the presence of indistinct fossil-hke forms, this formation has 

 been placed tentatively in the late pre-Cambrian or early Paleozoic. 

 The geological succession, therefore, is as follows: 



Mistassini limestones 



Unconformity ? 

 Diabase dykes 

 Cobalt series (includes Chibougamau formation ?) 



Great unconformity 

 Granite 



Intrusive contact 

 Lamprophyre dykes 



Intrusive contact 

 Sedimentary series (Pontiac series, Mattagami series, etc.) 



Unconformity 

 Gabbro and anorthosite 



Intrusive contact 

 Granite gneiss (around Lake St. John) 

 Intrusive contact 

 Nemenjish series 

 Abitibi volcanics (basalts, andesites, rhyolites) 



METHODS OF DETERMINING STRUCTURE IN LAVA FLOWS 



The lavas of the region afford exceptional opportunities for 

 structural determination, since they have suffered relatively little 

 deformation beyond a change in attitude. Each flow has moved as 

 a unit during the folding, so that shearing has been confined to a 

 marginal belt from one to ten feet in width. In the remaining 

 portions of the flows the original structures and textures remain 

 largely unchanged and can be used for determining the structure. 

 The lack of deformation is undoubtedly due to the thickness of 

 the flows, which has made them very competent to resist the de- 

 forming influences. The latter have been powerful enough to con- 

 vert the conglomerate, arkose, and greywacke of the overlying 

 Mattagami series into schist. 



Grain and pillow or ellipsoidal structure were the structures and 

 textures used in determining the attitude of the flows. Amygda- 

 loidal textures, so frequently useful for the purpose, are not found 

 here, since the margins of the flows are almost invariably schistose. 



