Some Points in the Geology of Northern Wisconsin. 109 



The completeness with which this drift covering conceals 

 the Laurentian rocks, may be understood, when I say that ia 

 all that portion of Ashland county underlaid by them, an area 

 of not less than 800 square miles, only four localities are known 

 where outcrops are to be seen, all of which are very near to 

 the junction of the overlying Huronian, and two of which are 

 caused by the action of rivers, the main Bad river and its 

 Marangouin branch, — where they leave finally the Laurentian 

 rocks and pass on to the Huronian. At each of these places the 

 exposures are bold, and give rise to falls of some size. Those 

 on the Marangouin have a height of sixty feet in three leaps, 

 the river curving as it falls, so that the last leap faces in a di- 

 rection at right angles to the first, the curve being around a 

 bold face of svenitic rock. 



The Huroyiian (II on map and section) rocks, which directly 

 overlie the Laurentian and unconformahly^ — as shown by 

 Brooks and Pumpelly, from observations made by them just 

 east of the Montreal river in Michigan — constitute in Ashland 

 county a continuous narrow belt, whose central portion is the 

 well known Penokie Range, and whose width never exceeds 

 two miles, being generally much less than this. These rocks ex- 

 tend without break into Michigan, almost as far as lake Goge- 

 bic, when they become lost, being covered by accumulations of 

 drift, and finally by newer rocks, until, one hundred miles east 

 of lake Gogebic, the Marquette Iron Region is reached, 

 where they are again found, but covering a much wider ex- 

 tent of country than in Wisconsin. Towards the west, the 

 Huronian belt comes to an abrupt ending, the underlying 

 Laurentian, and overlying Copper Bearing series closing 

 in on one another. Still further west, however, just on 

 the west side of Ashland county, are two isolated belts of Hu- 

 ronian rocks, in every way similar to the main area, having 

 each also its central ridge rising abruptly above the level 

 of the country. Further west still, we know nothing of 

 its continuance. In an old congressional document I find an 



