STRUCTURAL GEOLOGY. 21 



Distribution of the Archean 



made to outline the granites, either in the field-work or on the plates of vol. iv which 

 show the areal geology. Considerable areas of the northern part of the state have 

 not been traversed with sufficient frequency to furnish such information. It has 

 only been ascertained that in such areas the rocks in general pertain to the crystalline 

 series, and a single division has been, made, viz.: into schists and gneisses, and the 

 igneous granites have been included with the gneisses. In general, however, it is 

 true that large areas of Archean igneous granitic rock cross the northern part of 

 the state, trending with the grand features in a northeasterly direction. The Giant's 

 range is the most southerly of these belts. This forms the most abrupt Archean 

 hill-range in the state, running for the distance of fifty miles and maintaining the 

 prominence and often the sharp outline which simulates that of a great protruding 

 dike. Toward the north this granite spreads over the lower ground and disappears 

 under a sheet of drift so effectually that its northern limit is wholly conjectural. 

 This sudden change in topographic aspect suggests that the elevated belt is of a 

 different date from the low ground granite, and is perhaps of later date. The Animikie 

 lies in a tilted position on the southern slope of the Giant's range in its dike-like 

 form, as if it had been uplifted by the intrusion of that granite. 



Toward the northeast this belt of granite, after some interruption, is certainly, 

 in part at least, of Lower Keewatin age, as it supplied debris to the Upper Keewatin 

 conglomerates. This can be seen at the Kekequabic and Saganaga lakes, while at 

 Snowbank lake the granite of the region cuts the Upper Keewatin. 



The central islands and the southern shores of Bassimenan lake also consist of 

 the earlier granite, which seems to be continuous with the granite of the northern 

 shores of Burntside and Vermilion lakes, and with that of the international boundary 

 at Lac la Croix and Kabetogama. About the southern confines of Rainy lake, granite 

 gives place to much gneiss and schist. Granites are known in the central and 

 southwestern counties, as in Morrison, Stearns, Benton, Todd, Big Stone, Nicollet, 

 Redwood, etc., but the extent of these areas is unknown, owing to the prevalence of 

 the drift. 



Genesis of tiic Archean igneous rocks. The greenstones. The massive igneous 

 portions of the Lower Keewatin, the oldest known rocks in the state, are believed to 

 represent the first magma of the earth, and hence they are referable to the first 

 cooled crust that enveloped the molten mass. 



The question of the origin, as well as the order, of the Archean rocks has given 

 birth to much discussion, for it is one of fundamental importance. Much of the 

 diversity of opinion seems to have sprung from a defective and sometimes from an 

 erroneous idea of the rocks themselves, and from differences in the usage of terms. 

 The whole group of rock masses that lie below the basal conglomerate of the Taconic 



