SUMMARY OF CURRENT PRE-CAMBRIAN NORTH 

 AMERICAN LITERATURE.^ 



Gibson^ gives a summary, from the reports of the Canadian Geolog- 

 ical Survey, of the pre-Cambrian geology of the Hinterland of Ontario. 



Coleman^ gives a summary of the geology of the Rainy Lake region. 

 Following Lawson, the rocks are classified as follows : 



f Upper division, a. Keewatin. (Huronian ?) 

 ArcheanJ ^- Coutchiching. 



Lower division. Laurentian. 



The Laurentian rocks consist chiefly of granite-gneisses, with sub- 

 ordinate quantities of granite and syenite. The Coutchiching consists 

 of fine-grained mica-schists and mica-gneisses which show rapid changes 

 in composition in passing from one layer to another, thus suggesting 

 sedimentation. These rocks are usually sharply separated from the Lau- 

 rentian, but at Rice Bay the writer found himself in doubt as to the classi- 

 fication. The Coutchiching series is regarded as a metamorphosed sedi- 

 mentary one. As to the source of the material there is no very definite 

 information, unless certain gneisses in Sand Island river having layers 

 differing sharply in composition be looked upon as remnants of an 

 original Laurentian floor. The Keewatin is a series of eruptive and 

 fragmental rocks of great thickness and variety, consisting broadly of 

 a lower division of basic eruptives and volcanic ashes, and an upper 

 acid division. The bulk of the lower basic portion consists of diabases, 

 with some gabbros and anorthosites, and apparently some diorites. 

 Porphyroids are also present. The schistose members, often inter- 

 bedded with the massive altered eruptives, near the contact with the 



'Continued from p. 372, Vol. IV., Journal of Geology. 



^The Hinterland of Ontario, by T. W. Gibson. Fourth Kept. Ontario Bureau of 

 Mines, 1894, Sec. Ill, part on pp. 124, 125. Toronto, 1895. 



3 Gold in Ontario: Its Associated Rocks and Minerals, by A. P. Cole. Fourth 

 Rept. Ontario Bureau of Mines, 1894, Sec. II, pp. 35-100. Accompanied by two 

 geological maps of parts of the Rainy River district. Toronto, 1895. 



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