8,500 feet. 



1, 750 



(( 



1,800 



(( 



800 



u 



238 



for present use the profile will be taken along an ancient Indian 

 trail, that leads from the mouth of Montreal River to Lac Flambeau, 

 and the formations will be numbered 1, 2, 3, etc. 



The provisional arrangement, which it is always necessary to make 

 in the early stages of the examination of a new region, must of course 

 be based upon the external characters of the rocks. In this case it is 

 not only convenient, but proves to be a correct arrangement. 



The following is a general view of the structure of the formations 

 in the descending order : — 



Formation No. 1. — Potsdam Sandstone. 

 On the Montreal River, strike northeast by east, in places N. 60° 



E. ; dip northwest by north, 75° to 90°. It embraces four members, 



a, 6, c, and d. 



a. Sandstone Proper^ corrected for bevel, thickness, 

 &. Alternations of sandstone and black-slate, thickness, 



c. Conglomerate, thickness, 



d. Alternations of trap and sandstone, thickness, 



Total 11,850 " 



This is not the entire thickness of the Potsdam at the mouth of the 

 Montreal. The synclinal line lies an unknown distance out in the 

 lake, perhaps one-fourth of a mile, and whatever this distance may 

 be should be added to the above statement. At the Apostle Islands 

 and in Chegoimegon Bay the dip is reversed, having a direction to- 

 wards the southeast ; but the line along which the change occurs is 

 covered either by the waters of Lake Superior or by drift. On the 

 northerly side of the synclinal the plunge of strata is much less in 

 amount. It presents the case of nearly horizontal beds on one side, 

 and of nearly vertical ones on the other. Following the outcrop 

 along the southerly shore to the west end of the lake, and thence 

 along its northwesterly coast, the dip is everywhere conformable, and 

 to the southeast. By estimates and measurements on that shore, com- 

 bined with those at the Montreal, I regard its total thickness to be 

 not less that fifteen thousand feet. No fossils have yet been found in 

 the sandstone of the west end of Lake Superior. Its color is gen- 

 erally red, owing to the presence of oxide of iron. Where this is 

 wanting, it is gray or a dull white, and in places mottled gray and red. 



Formation, No. 2. — Trappose, in two members. 



a. Brown amygdaloid ; dip and strike conformable to . 

 formation 1 ; thickness along Lac Flambeau trail, 3k miles. 



b. Compact red and blue 2^ " 



Total 6 « 



