Il8 THE JOURNAL OF GEOLOGY. 



in the region of Rainy Lake and Lake of the Woods, northwest 

 of Lake Superior. If this conclusion be true, the sea did not 

 advance as far as the Lake of the Woods, this district perhaps 

 being above the ocean, and one of the sources of detritus through- 

 out Lower Huronian time. 



The extent of the Lower Huronian deposits is also uncertain. 

 If the series of the districts above placed in the Lower Huronian, 

 are correctly correlated. Lower Huronian basins occurred in vari- 

 ous places over a great triangular area extending from Black 

 River Falls in Wisconsin, to northeastern Minnesota, and thence 

 east to the north shore of Lake Huron. Doubtless Lower 

 Huronian rocks also occur in the great northern region of 

 Canada, and they may have had a much wider original extent 

 than this, but no data are now available to locate such a possible 

 extension. 



Of the original thickness of the Lower Huronian deposits we 

 are also ignorant. The present thickness has not been deter- 

 mined south of Lake Superior, but according to Logan, on the 

 north shore of Lake Huron, including the interstratified volcanics, 

 the thickness is five thousand feet. 



At the end of Lower Huronian time, the Lake Superior 

 region was raised above the sea, folded, and subjected to erosion. 

 The orographic movements of this time were very severe, closely 

 crumpling in places the rocks of the Lower Huronian, and induc- 

 ing in them in many places a schistose structure. In other locali- 

 ties, away from the axes of great disturbance, the Lower 

 Huronian rocks were but gently tilted, as is shown by the small 

 discordance in places between them and the succeeding series. 

 In certain localities the areas of great disturbance are but a short 

 distance from those of comparative quiet. The denudation was 

 deep enough to wholly remove the entire series over wide areas, 

 and to cut to unknown depths into the Basement Complex itself. 

 As has been stated, the Lower Huronian has an estimated thick- 

 ness of about one mile on the north shore of Lake Huron, and 

 in different localities varies from this thickness to entire absence, 

 depending mainly upon the differing deundation. This variabil- 



