STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 275 



The age of the anorthosite, while not definitely fixed, may 

 tentatively be placed as post-Grenville and pre-Mattagami. In 

 the Lake St. John district it has been found to intrude the Gren- 

 ville, and also to intrude an earher granite which also intrudes 

 the Grenville. It is itself intruded by the later granite that cuts 

 the Mattagami series. It has never been found in direct contact 

 with the Mattagami series. A mass of anorthositic gabbro on 

 the Bell River intrudes the Abitibi volcanics but does not rise 

 as high as their contact with the Mattagami series. In the Lucky 

 Strike area a single pebble of what was supposed to be anorthosite 

 was observed in the basal conglomerate of the Mattagami series. 

 This pebble could not be got out of the matrix for microscopic 

 study, however, so that its exact determination is uncertain. The 

 age of the anorthosite can, therefore, only be placed tentatively 

 as pre-Mattagami. 



GRANITES 



In the vicinity of Lake St. John, J. A. Dresser^ has reported 

 the presence of granites both earher and later than the anorthosite 

 there. To the northwest of the lake all the granite observed is 

 later than the anorthosite. To the south and east of the lake, 

 Adams^ has reported that the granite is earlier than the anortho- 

 site. In New York state, Gushing, Kemp, and Smyth state that 

 the anorthosite is later than the gneisses which intrude the Gren- 

 ville series. The existence of granites of two ages therefore seems 

 to be fairly definitely estabhshed. The earlier of these seems to 

 have been confined in its distribution to the south and east of 

 Lake St. John; in age it postdates the deposition of the Grenville 

 series and antedates the intrusion of the anorthosite masses. The 

 later granite was intruded mainly throughout the region to the 

 north and west of Lake St. John, though it is also found in small 

 amount to the south and east. In age it is pre- Cobalt, but is 

 later than the anorthosite intrusions and the deposition of the 

 Mattagami series. 



' Geol. Siirv. Can., Mem. No. g2, 1916. 



=• Geol. Surv. Can., Ann. Kept., New Ser., VIII (1897), Part J. 



[To be continued] 



