STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 269 

 NEMENJISH-GRENVILLE SERIES 



A series of rocks which are the metamorphosed equivalents 

 of clastic sediments has been termed the Nemenjish series in the 

 foregoing pages. They attain their largest extension in the contig- 

 uous Windy Lake and Nemenjish areas. These rocks are believed 

 to be the northern equivalent of the Grenville series. 



The primary basis for this tentative determination is petro- 

 graphic. It is recognized clearly that petrographic similarities as 

 a rule form a most insecure foundation for correlation; but the 

 petrographic pecuHarities of these rocks are so marked and so 

 different from those of any other rocks observed by the writer 

 in the pre-Cambrian of Ontario or Quebec that they afford unu- 

 sually good evidence for this purpose. The major portion of the 

 rocks, above the basal beds, are highly garnetiferous. They con- 

 sist of rather fine-grained, Hght grey mica gneisses or schists, and 

 greenish-black hornblende gneisses, crowded with small, light, pink 

 garnets. The garnets in the mica gneiss are usually small, 1-2 

 mm. in diameter; those in the hornblende gneiss usually some- 

 what larger, 3-5 mm. in diameter. Garnetiferous types probably 

 average at least jo per cent of the total bulk. No such bulk of garnet- 

 iferous rocks is found in any other pre-Cambrian series. Gar- 

 netiferous types, usually garnetiferous hornblende schists, occur 

 locally in the altered basic lavas, often termed "Keewatin.'' and 

 also in the Mattagami series, but invariably in minor amount, 

 less than i per cent. The writer therefore considers this criterion 

 as a rehable one for correlation purposes, until it shall have been 

 shown that another formation with similar bulk characteristic 

 exists in the pre-Cambrian shield. 



The Grenville series extends north of the Ottawa River at 

 Ottawa for approximately 140 miles, almost to the junction of 

 the east and west branches of the Gatineau River. It is bounded 

 on the north by a great bathoHth of granite intrusive into it. A 

 section made by the writer in 19 16 down the Gatineau River 

 from the National Transcontinental Railway to Ottawa showed 

 that within the boundaries mentioned above the continuity 

 of the series is unbroken, except by relatively small granitic 



