84 



SKETCHES OF CUE ATI OX. 



The convulsions which closed the Potsdam period pro- 

 truded through this formation enormous vertical walls of 



molten rock, known 

 by the general name 

 of "trap," or dolerite, 

 or delessite. The 

 bursting through of 

 these igneous mate- 

 rials tilted up the 

 broken edges of the 

 Outlines of Lake Superior. sandstone, and form- 



ed between the lines of outburst deep valleys, which have 

 become the bed of the lake. The sandstone which plunges 

 beneath the water's surface on the northwest side of Kew- 

 enaw Point reappears on He Roy ale, which was formed, 

 like Kewenaw Point, by an outburst of dolerite (Fig. 31). 



Fig. 



Fig. 31. Section across Lake Superior, along the line XX, Fig. 30. 

 a, a. The water level, b. Trap outburst north of the lake. c. Trap outburst form- 

 ing Tie Koyale. d. Trap outburst forming Kewenaw Point, e. Lake Superior 

 sandstone and conglomerate, f. Eozoic and other rocks underlying the sand- 

 stone. 



From the north side of He Royale the sandstone glides 

 under the water again, and reappears upon the northern 

 shore of the lake. The basin of the lake is therefore a ge- 

 ological valley — a " synclinal" valley — formed by the ig- 

 neous eruptions upon the northern and southern shores. 

 Its origin, as will be seen, is entirely different from the 

 origin of any of the other lake basins of the chain. 



The escape of the molten rocks of the region fused out 

 the copper and silver, which were disseminated through 

 the neighboring strata, and accumulated them in masses 

 of great commercial importance. An enormous dike of 



