340 W. II. C. SMITH — A.RCHEAN ROCKS WEST OF LAKE SUPERIOR. 



THE CONTCHICHINO SERIES. 



Rocks composing it. — The < lontchiching series consists essentially of finc- 

 grained evenly laminated biotite gneisses, light gray in color : of fine to' 

 coarsegrained mica schists, generally highly feldspathic and sometimes 

 very quartzose, from dark gray to light gray in color and brownish or 

 " rusty " weathering, and of fine-grained hornblendic mica schists. 



Position and Relation ofitsRocks. — In the southeastern part of the district 

 they occupy a position always intermediate between the granite gneisses, 

 which in the contact zone generally invade them in parallel bands, 

 apophyses and dikes, and the hornblende schists and altered traps at 

 the base of the Keewatin, which overlie them in conformable position. 

 They frequently merge into these by a gradual change in mineral com- 

 position across the strike. N. H. Winchell * refers to a gradual and con- 

 formable transition between Vermiline (Contchiching) and Keewatin. 



In the Lake of the Woods there is a series of mica schists which, 

 according to Dr Lawson's descriptions^ are closely similar to those which 

 he subsequently separated from the Keewatin and designated under the 

 name of Contchiching. In his hypothetical sections of this district, the 

 accuracy of which, however, he does not insist upon, he relegates these 

 mica schists to the highest position in the Keewatin scale. By a refer- 

 ence to the map, however, it will be seen that in their most important 

 development they occupy a position on the margin of the Keewatin 

 trough in the southwestern part of the lake and in direct contact with the 

 Laurentian granites. While developments of these mica schists are 

 found in interior portions of this Keewatin trough, such a position may 

 easily be accounted for on the assumption that they represent the crests 

 of anticlinal folds exposed by denudation. Indeed, the foldings and 

 disturbances in this trough have been so great that almost any position 

 in the scale may be attributed to the mica schists of this complex series. 

 The inference is strong that these rocks in the Lake of the Woods are 

 the equivalents of the Contchiching series of Rainy lake. 



Its Thickness. — In this latter region Dr Lawson attributes to the Cont- 

 chiching series;j; a maximum thickness of from 24,000 to nearly 29,000 

 feet. On a similar interpretation of the structure east of tins, in the 

 western part of the Hunters Island region, about the same thickness may 

 lie inferred. The writer, in bis report on this latter region, now in press, 

 states at some length bis reasons for doubting that these rocks have such 



* 17th Ann. K.'p <>t' the Geol and Nat. Hist. Survey of Minnesota. 



f Report on the (ieology of the Lake of the Woo Is, by Andrew C. Lawson. Pari C E of the Ann. 

 Rep. Geol. Surv. of Canada, 1885. 



; Report "ii the Geology of the Rainy Lake Region, by Andrew C. Lawson, M. A., Ph. L>.. part E, 

 Ann. Rep. Geol. Surv. of Canada, L887 '88. 



