STRUCTURAL GEOLOGY. 19 



The Archean greenstones.] 



end of Birch lake, at the west side of White Iron lake, on the Kawishiwi river and 

 at West Seagull lake. When the granite is in large masses the greenstones are 

 metamorphosed by general and dynamic forces and converted to hornblende schists 

 and amphibolytes. This alteration is not due so much to actual contact of the 

 granite as to some deep-seated and widespread force acting on the older rocks over 

 extended areas at the time of the granite intrusion. The older rocks seem to have 

 been nearly at the same temperature as the intrusive rock, since the dikes do not 

 generally show the structure due to chilling along their borders. When such dikes, 

 however, are remote from the centres of metamorphism they are seen to be chilled 

 at their borders. 



As the greenstones merge into more and more clastic and siliceous rocks, as 

 seen about the Twin peaks, and at the northwestern confines of Vermilion lake, and at 

 Rainy lake, such widespread metamorphjsm gives rise to banded gneissic rocks of 

 varying composition, sometimes quite hornblendic and sometimes quite siliceous, 

 and in this form the upper part of the Kawishiwin has apparently been extensively 

 altered into a group of rocks (gneisses, mica schists, etc.), which have very generally 

 been taken to be the oldest known rocks of the earth's crust. 



Besides these basal greenstones and their attendants, of the age of the Lower 

 Keewatin, there are similar greenstones in the Upper Keewatin. These, however, 

 differ from the older in being in general more plainly fragmeiital, and often in 

 containing coarse fragmental debris. These fragniental beds are, in part at least, the 

 rocks sometimes called "slate cougomerates." They contain sometimes large pebbles 

 of diabase and of granite, and often are distinctly bedded, making, when they become 

 finer, graywackes, slates and quartzytes. That volcanic action was continuous into 

 the Upper Keewatin, in northern Minnesota, is in harmony with all the structural 

 and petrographic phenomena of the Upper Keewatin rocks, but exactly where the 

 vents of volcanic ejection existed, it is at present impossible to state. It is probable 

 that the volcanic areas of Lower Keewatin time would some of them be perpetuated 

 into Upper Keewatin time, the intervening revolution not being sufficient to perma- 

 nently obstruct the old ducts. It is hence also probable that it would be very 

 difficult to divide the greenstones produced by such prolonged action in the near 

 vicinity of such vent, into two epochs comparable to the two epochs into which the 

 Keewatin fragmentals are divided. The region of the Twin peaks south from Ogishke 

 Muncie lake is likely to be the seat of a long continued volcanic vent. Another 

 seems to have existed about the northeastern-confines of Kekequabic lake, and still 

 another about the eastern end of Otter Track lake on the international boundary. 



The granite of the Arc/tea it are well known. Several are described in later 

 chapters in this volume, and their structural relations to the schists, gneisses and 



