202 H. C. COOKE 



east of the most eastern outcrop of the Pontiac series. The inter- 

 vening distance has not been closely examined. It seems strange 

 that the Grenville series should approach the Pontiac so closely and 

 yet not be found within the Pontiac area, and that so widespread 

 a series as the Pontiac should end so sharply, and no further trace 

 of it be found included in the granites to the south or east. Both 

 these difficulties would at once disappear, however, if at least a 

 part of the Pontiac were identical with the Grenville series. 



4. To the north of the main belt of the Pontiac series Wilson 

 has mapped several minor areas of sediments surrounded by the 

 Abitibi volcanics. They are of mica schist only, without the con- 

 glomerate member. If the conglomerate and all the mica schists 

 form a part of the same series, the conglomerate member should 

 appear in these areas also, along the line of contact of the green- 

 stones and the sediments. 



In \dew of these facts the writer tentatively advances the view, 

 alternative to that suggested by Wilson and Bancroft, that the 

 so-called Pontiac series consists of two sedimentary series. The 

 older, composed mainly of mica schists and amphibolites, is equiva- 

 lent to the Grenville and Nemenjish series and may lie upon the 

 older volcanic series conformably. The younger contains the con- 

 glomerate, arkose, and greywacke members, and perhaps a portion 

 of the mica schists, and may be equivalent in age to the Mattagami, 

 Broadback, Lucky Strike, and Brock series. 



The rocks of the volcanic complex underlying the Pontiac series 

 consist mainly of lavas varjdng in composition from basalt to 

 rhyolite; the t^pes of intermediate composition are the most com- 

 mon. Interbedded with them are small amounts of slate or slaty 

 tuff, and dolomite, which in part at least is an altered rhyolite. 

 Intrusive into these rocks are bodies of gabbro and diabase. All 

 of the rocks have undergone more or less metamorphism and have 

 been converted locally into schists. Outcrops throughout the area 

 are too sparse to determine structure and succession, but wherever 

 strikes are determinable they have a general east-west direction. 



Summary. — The geological sections described in the foregoing 

 areas mav be summarized as follows: 



