THE GEOLOGY OP MINNESOTA. 



[Distribution of the fragmental rocks. 



rocks. But the coarseness of the debris, as well as the great thickness of the 

 deposits, which in the Upper Keewatin reaches nearly two miles, attests the violence 

 of the waves and currents which operated to produce and to transport the materials. 

 In some places these materials are largely from the igneous rocks of the older for- 

 mations, as in the Stuntz conglomerate, and in others they were derived from former 

 clastic rocks. In some cases these fragments are coarse and angular, denoting pow- 

 erful destructive agents but feeble transporting. 



Distribution of the frag mentals of the Archean. The most important belt of the 

 fragmentals of the Archean is found in the northeastern part of the state, extending 

 northeastward, from Vermilion lake to Saganaga lake. It belongs to the Upper 

 Keewatin and it lies non-conformably on the Lower Keewatin. Its materials every- 

 where contain fragments from the underlying, older strata. It occupies, apparently, 

 the basin of an ancient syncline whose axis, in Minnesota, is traceable from 

 Vermilion lake to the international boundary at Saganaga lake. The arms of the 

 original syncline consist of granite and gneiss, and the origin of the fold probably 

 dates from the appearance of the granite concerned. The northern arm comprises 

 the granites and gneisses of the northern part of Vermilion lake, with the associated 

 mica schists; these extend eastward by way of Bassimenan lake and enter Canada 

 on the northern side of Hunter's island. Throughout some portions of this arm the 

 igneous basic rocks, earlier than the granite, rise higher than the granite, and this is 

 particularly the case on the southern side of Hunter's island. In like manner, the 

 southern arm of this main synclinal valley consists of the early igneous Keewatin 

 and its associated green schists and other schists, penetrated by a great range of 

 granite which in its southwestern extension is known in part as the Giant's range. 

 This granite belt, with its accompanying gneiss and mica schist, seems to come from 

 far toward the southwest, but owing to the prevalence of the drift, it cannot be 

 traced with certainty any further west than Pokegama falls, on the Upper Missis- 

 sippi. Toward the northeast this arm sinks away and it is encroached on by the 

 gabbro on the south, while the granite itself gives place to a characteristic massive 

 greenstone along the north. In this way the axis of the depression has been shifted 

 toward the south, and the overlying Upper Keewatin is brought, at Saganaga lake, 

 immediately upon the granite itself, which further southwest constitutes the most 

 of the southern rim of the basin. In this trough lie the rocks which, typically and 

 specially, are herein designated Upper Keewatin. The rocks of the Lower Keewatin, 

 mainly ferro-magnesian in character, whether massive or fragmental, and the later 

 granite form the slopes and the summits of the rim. In general this expresses not 

 only the distribution but the structure of the Archean lying to the north of the 

 Cabotian gabbro and the Animikie, in the northeastern part of the state. There are 



