190 H. C. COOKE 



and along the central synclinal axis there lies a small area of basic 

 tuffs, overlain by twenty to thirty feet of fragmental rhyolite por- 

 phyry. The succession was well determined, and the distribution 

 would indicate conclusively a general synclinal structure were it 

 not suspected, as in the Opawika area, that the rhyolites are 

 localized in their original distribution. 



The best determinations of structure were made on the western 

 part of Obatogamiau Lake and the eastern part of Eau Jaune Lake, 

 where the presence of large and easily determinable ellipsoidal 

 structures rendered work easy. Time did not permit of the inten- 

 sive w^ork necessary for the determination of the structures of the 

 basalts in the northern and eastern parts of Obatogamau Lake, 

 while outcrops are rare along the eastern shores of Eau Jaune Lake. 



The basalts of Obatogamau Lake are characterized by ellip- 

 soidal structures, in contrast with the basalts of the areas to the 

 west. The ellipsoidal structures in the basalts are small and diffi- 

 cult of recognition, except on unusually clean surfaces, so that they 

 do not aid greatly in the determination of the attitudes of the flows. 

 If this structure is an indication of submarine extrusion, however, 

 their occurrence at this point will indicate that the seashore at the 

 time of the basaltic extrusion lay between Obatogamau and Kaopa- 

 tina lakes. From this point it must have transgressed rapidly 

 westward as extrusion proceeded. 



The succession as determined in this area is : 



Rhyolite porphyry, fragmental Andesite flows 



Andesite tuffs Basalt flows 



Brock area. — The Brock area (Fig. 8) is situated at the con- 

 fluence of the Brock and Chibougamau rivers. Unfortunately out- 

 crops are poor in this locality on account of the cover of stratified 

 sands extending up the Chibougamau valley. 



The basal rocks are a series of basic lavas which are here more 

 schistose than those farther to the south, presumably because of 

 the shear between them and the overlying formation at the time 

 of folding. Their structure was not determined. To the north of 

 these is a series of sediments, which were found to form the south 

 limb of a syncline, the north limb of which has been cut off by 

 granite. At the southern contact of the series lies a heavy band 



