274 H. C. COOKE 



rhyolitic lava or tuff/ Wherever sediments of Nemenjish age 

 have been found the lavas underlie them conformably. Where- 

 ever patches of the Mattagami series occur, they overlie the lavas 

 with unconformity. Wherever anorthosite masses outcrop, they 

 intrude the lavas. 



As these lavas, like the Mattagami series, underlie the great 

 erosional peneplain that underlies the Cobalt series, it is possible 

 that they are to be correlated with the lavas in the Cobalt district, 

 which are there known as "Keewatin" and which occupy a similar 

 stratigraphic position. There is as yet, however, little or no 

 evidence for correlating these lavas with the Keewatin of the north 

 and south shores of Lake Superior. The local name of Abitibi 

 volcanics will therefore be extended to apply to the lavas in this 

 whole region. This name was first applied by M. E. Wilson in 

 1909 to the lavas in the Kewagama map-area, in the southwestern 

 part of this region. 



ANORTHOSITES 



Anorthosite and anorthositic gabbro do not appear to any 

 large extent within the region under discussion. A mass of con- 

 siderable size is found on the Bell River, near Mattagami Lake, 

 another on Chibougamau Lake. Smaller masses are found on 

 the Opawika and Chibougamau rivers; there is an immense mass 

 in the vicinity of Lake St. John. To the east and south of Lake 

 St. John, the amount of anorthosite is much larger; it is also 

 present in large amount in the Adirondacks. 



Since anorthosite may be formed as a differentiate of any 

 gabbroid magma under suitable conditions, and since it so happens 

 that the anorthosite masses are not as a rule in contact with a 

 sufficient number of sedimentary formations for their age to be 

 closely determined there is Httle or no evidence that they may 

 not be of more than one age. However, the occurrence of such 

 great masses of anorthosite has been rare throughout geologic 

 history. Hence it is considered highly probable that all of the 

 anorthosite belongs to a single period of igneous activity and owes 

 its origin to some common cause. 



^ It may be here mentioned that the writer's work of 191 7-18 has shown that the 

 simple succession described for the lavas of northern Quebec does not prevail in the 

 Montreal River district of northern Ontario. 



