334: GEOLOGY OF EASTERN WISCONSIN. 



and still farther on, it is reduced to four feet six and one- half inches. 

 Near the point of greatest thickness, there are slight indications of 

 "taking on" layers. The ore at this point is very similar to that at 

 Iron Ridge. The oolitic concretions are somewhat larger on the aver- 

 age, and of a slightly duller color, and there is a greater amount of 

 argillaceous material present. Upon the bed of lenticular ore, there is 

 a highly pyritiferous layer that seems to represent the dark layer at 

 Iron Ridge. Along the shore of Green Bay, south of Little Sturgeon 

 Bay, this geological horizon is marked by a somewhat continuous bed 

 of ore, or ocherous rock, of a few inches thickness. It very rarely pre- 

 sents an oolitic structure, or other characteristic of the deposits already 

 described, but it is identical with them in geological position, and 

 must be regarded as their stratigraphical equivalent. 



It appears, then, from the foregoing facts, that this iron ore occurs 

 at widely separated points, and that between these it is entirely want- 

 ing, or is represented only by a rusty seam between the limestone and 

 shale. There can be no doubt that it was a marine deposit, for in ad- 

 dition to the evidence of the beds in this state, the equivalent forma- 

 tion elsewhere contains the remains of marine life. It appears alto- 

 gether probable that the ore was deposited in detached basins, over 

 which, and over all the intervening region, the Niagara limestone was 

 afterwards laid down, enclosing the detached deposits between it and 

 the shale below in the form of lenticular masses. This ore is to be 

 sought for only between the Cincinnati shales and the Niagara lime- 

 stone. The line along which the junction of these two formations 

 comes to the surface w r ill be found traced with much care on the 

 accompanying maps. To the east of this line, the horizon of the ore 

 lies beneath the Niagara limestone, and, so far as geological evidence 

 goes, is as likely to be found at the base of that formation^, at any 

 point over the broad area occupied by it, as at the points where its 

 base has come to be exposed at the surface by denuding agencies. Of 

 course the value of a bed of ore, found at any considerable depth below 

 the surface, would be very seriously affected by its unfavorable po- 

 sition. 



While it is probable that beds of this ore do exist at points not yet 

 discovered, it is prudent to bear in mind that deposits of the thick- 

 ness of that at Iron Ridge are very rare, there being but a single 

 known instance of a thicker deposit in all the wide range of the Clin- 

 ton formation, though beds of less depth are profitably mined; and 

 while it is legitimate to hope that further profitable discoveries may 

 yet be made, it is the part of wisdom to exercise due caution in the 

 expenditure of time or- funds in exploration, and to be guided by an 



