Some Points in the Geology of Northern Wisconsin. 119 



produce all the phenomena observed. Such a movement is not 

 at all difficult to explain, 



IT. Westward continuation of the Lake Superior synclinal. 



Foster and Whitney long since pointed out that the rocks 

 of Isle Royale and those of Keweenaw Point formed the oppo- 

 site edges of a synclinal in whose depression lie the waters of 

 Lake Superior, The rocks of Keweenaw Point continue west- 

 ward uninterruptedly as far as Long Lake in Wisconsin, Do 

 those of Isle Eoyale extend westward in a similar manner? I 

 have obtamed facts going to show that they do, and that the 

 peninsula of Bayfield owes its existence to this extension. 

 The facts alluded to are these : 1st, The known existence of 

 a synclinal in Ashland county. 2d, The southward dip of 

 traps of Douglas county. 8d. The known existence of an ex- 

 tension of the Douglas county traps into Bayfield county."' 4th, 

 The vertical position of the beds of traps and sandstone in the 

 westward continuation of the Keweenaw Point belt, as com- 

 pared with the much lower dip at its eastern end ; — since to- 

 wards the west the two sides of the supposed synclinal ap- 

 proach more nearly to one another, it would be expected that 

 the dip would be much greater, on one side at least, than 

 where they are far apart, 5th. A westward continuation of 

 the IsIeRoyale rocks is nowhere else found. 6th. The Doug- 

 las county traps are very similar to those of IsIeRoyale. The 

 accompanying section shows at once what is actually known of 

 the geology of the country along its line (D. E. on accompany- 

 ing maps), and the probable structure of the concealed portions. 



The views thus set forth at once explain, and are confirmed 

 by, the ferruginous and aluminous character of the Lower 

 Silurian sandstones of Lake Superior, as compared with the 

 quartzose character of their equivalents in the Mississippi 

 valley ; the former have been derived from the wear of the ftld- 

 spathic and magnetite hearing trajjs of the Copper -bearing 

 seriesj whilst the latter owe their material to the wear of quartzose 

 Laurentian granites andof Huronean quartzites and scJusts. 

 ♦These traps dleappear under the drift as they are traced eastward. 



