STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 73 



G. A. Young. "Lake St. John District," Geol. Siirv. Can., Ann. Rept., 1900, 



Part A, p. 143. 

 J. A. Dresser. "Lake St. John District," Geol. Surv. Can., Mem. No. g2, 1916. 

 0. O'SuUivan. "Explorations Along the National Transcontinental Railway 



from La Tuque Westward," Geol. Surv. Can., Sum. Rept., 1907, p. 67. 



GENERAL GEOLOGY 



The oldest rocks of the district are lavas, tuffs, and sediments. 

 The bulk of the lavas are basalts and andesites, but more acid 

 types, such as rhyolite porphyrites, are also present, though only 

 in relatively small amount. Like the flows the tuffs vary in com- 

 position, and are of both coarse and fine grain. The flows and tuffs 

 have been locally metamorphosed by dynamic or thermal agencies. 

 They are probably to be correlated with the so-called "Keewatin" 

 of Timiskaming district to the southwest; that they are of the 

 same age as the Keewatin of the Lake Superior district has not 

 yet been established. 



The sediments associated with the volcanics are all more or 

 less recrystallized and are now chiefly represented by garnetiferous 

 and non-garnetiferous biotite gneisses, garnetiferous and non- 

 garnetiferous hornblende gneisses, and crystalline limestones. 

 These rocks probably are the metamorphic equivalents of impure 

 sandstones, shales, and limestones. The original nature of the 

 hornblende gneisses is not fully established, but from the entire 

 absence of any evidence of igneous origin they are tentatively 

 assumed to have been sediments. The clastic beds grade downward 

 into beds of more tuffaceous composition, which lie conformably 

 on the surface of the lavas. Evidence has been obtained that seems 

 to indicate that these sediments, which have been termed the 

 Nemenjish series, form the northern equivalent of the Grenville 

 series of eastern Ontario and the Adirondacks. 



In the vicinity of Lake St. John a granite is reported by J. A. 

 Dresser to intrude the Grenville series and to be cut in turn by 

 anorthosite. This granite has not been definitely shown to occur 

 anywhere else throughout the region under discussion, but the 

 reports of Adams, Gushing, and others suggest that it is found in 

 areas to the east and south. 



