382 E. C. COOKE 



SUMMARY 



This paper first describes methods which the writer has used 

 for the determination of structure in the volcanic flows, often 

 termed "Keewatin," that form the oldest known rocks of northern 

 Quebec. These methods, applied to four areas of these volcanics 

 on the Opawika River, a tributary of the Nottaway, have resulted 

 in the determination of a similar sequence of extrusion in each area. 

 The sequence is one of increasing acidity, and consists of basalt 

 at the base, overlain successively by feldspathic basalt character- 

 ized by large phenocrysts of feldspar, andesite, dacite, and rhyolite. 

 All members of this series are not necessarily present in any one 

 locality, but their succession when present is invariable as stated. 



The paper also establishes the occurrence in northern Quebec 

 of two important sedimentary series, both apparently of pre- 

 Huronian age. One of these, the Nemenjish series, has been found 

 by the writer in one locahty only. It consists of a series of garnetif- 

 erous gneisses, mainly the recrystalHzed form of impure sand- 

 stones, which still exhibit bedding and other characteristics of 

 sediments. The rocks rest with structural conformity on the sur- 

 face of the ancient lavas, and some evidence is given to show that 

 they probably are to be correlated with the Grenville series. The 

 other sedimentary series, the Mattagami, has been found in five 

 different areas. Evidence based on the character of the sediments 

 in each area, the sequence of the formations in each series, the struc- 

 tures, and the relations to older and younger rocks, is cited to show 

 that they are all of one age. The evidence relating to their position 

 in the geologic column is less satisfactory, but they appear to overKe 

 the Nemenjish series with unconformity, and to be much older than 

 the Lower Huronian, or Bruce series. 



In addition to these matters of principal interest, the paper 

 discusses a number of theoretical considerations. One of these 

 may particularly be mentioned. The explorations of northern 

 Quebec show the existence of a fairly sharp boundary line, on one 

 side of which Hmestone is a member of the Grenville series, while 

 on the other side it does not appear. The significance of this line 

 is discussed, and the conclusion reached that it represents an ancient 

 topographic break, either a shore line or the boundary of a sub- 

 merged plateau. 



