342 W. II. C. SMITH— rAECHEAN HOCKS WEST OF LAKE SUPERIOR. 

 CONDITIONS UNDER WHICH CONTCHICHINQ AND KEEWATIN WERE FORMED. 



The close of the period during which the Contchiching rocks were laid 

 down ushered in an era of intense and long-continued volcanic activity. 

 interrupted perhaps and succeeded by periods of compensative quies- 

 cence during which erosion and sedimentation took place; but during 

 Keewatin times no certain evidence of any great or extensive crustal 

 movements is afforded. The whole Keewatin and Contchiching series 

 seems to have been folded by one great and perhaps simultaneous up- 

 heaval of the original floor. This folding marked the close of the 

 Keewatin epoch. Barlow* suggests that the concentric lamination in 

 the ovoid areas of granite gneiss indicates that the forces of upheaval 

 acted from certain centers ; this may he so, hut the phenomenon may 

 also be accounted for by the flow of the magma being directed by its 

 proximity to the hard schists, it may be more correct to say that the 

 folding of the schists was caused not so much by an upheaval of the sub- 

 crustal magma acting from centers of force as by the crumpling, due to 

 lateral compression, forcing their synclinal folds into the plastic magma. 



THE KEEWATIN SERIES. 



Rocks composing it. — The Keewatin series consists for the most part of 

 plutonic, volcanic and pyroclastic rocks. While some of the upper 

 members seem to be more or less altered aqueous sediments, the propor- 

 tion of Undoubtedly clastic rocks is small. 



Its stratigraphic Succession. — Unfortunately the microscopic study of 

 these rocks is as yet incomplete. The solution of their stratigraphic suc- 

 cession is confronted by almost insurmountable difficulties, and only a 

 general suggestion as to the sequence of broad and ill-defined groups can 

 be offered. The line of demarkation between the numerous horizons is 

 seldom clear, and where those horizons can be separated at all they are 

 not always found to occupy the same relative position. They are seldom 

 very persistent, and overlap each other as more or less attenuated lenticu- 

 lar hands. I know of no place in this district presenting a complete sec- 

 tion of the Keewatin. The most important and complete development 

 of this series is found in the Lake of the Woods and Rainy Lake regions. 



Speaking generally, then, the basal members of this great series con- 

 sist of dark green or black crystalline hornblende schists, generally fine- 

 grained, and which are sometimes seen to merge into the mica schists of 

 the Contchiching : of dark and light green altered traps, generally mas- 

 sive, hut sheared and broken by pressure and sometimes rendered 

 schistose; of green chlorite schists, which sometimes seem to be altered 

 hornblende schists and often again are almost undoubtedly hut highly 



* Am. i reologist, vol. n i. qo, 9, July, 1890. 



