370 H. C. COOKE 



area of garnetiferous mica gneisses, intruded by granites. The 

 occurrence of such sediments is of great interest, as indicating the 

 possibility of future correlation with the eastern side of Hudson 

 Bay. 



THEORETICAL CONSIDERATIONS 



Conditions of Extrusions of Ahitihi Vole anics 



The nature of the floor upon which the Abitibi volcanics were 

 poured out cannot be more than a matter for conjecture. No trace 

 has as yet been found of the rocks which once composed it, to show 

 whether they were sedimentary or igneous, flat-lying or folded. 

 Other facts are, however, more certain. 



In the following section the conclusion is drawn from a considera- 

 tion of the nature of the Grenville sediments that the Grenville 

 series in the interior of Quebec was deposited under continental or 

 shoal- water marine conditions. The region was therefore a con- 

 tinental plateau, or positive element, even at this time. As 

 evidence is lacking of the occurrence of any great earth movement 

 between the extrusion of the Abitibi volcanics and the deposition 

 of the Grenville series, it would be reasonable to conclude that 

 during the extrusive period also the region must have been a positive 

 element. Some direct evidence on this question is obtainable from 

 the lavas themselves, using pillow lavas and bedded tuffs as criteria 

 of subaqueous extrusion. 



On Obatogamau Lake all the lavas, even the basal basalts, 

 show pillow structures. On Kaopatina Lake and eastward to 

 Father's Lake, the basalts are not pillowed, but the overlying lavas 

 have the structure well developed, and interbanded tulTs are well 

 bedded. On Tush Lake and the areas to the northward, nona of 

 the lavas are well bedded. While this line of evidence has not been 

 worked out in detail throughout the entire area, the facts obtained 

 suggest that the. extrusive period was one of gradual submergence. 

 The earliest recognized shore must have lain between Obatogamau 

 and Kaopatina lakes, and a westward transgression of the sea 

 took place thereafter. The submergence evidently went on after 

 the extrusion of the lavas was complete, since garnetiferous gneisses 



