246 APPENDIX. 



the upper beds of the cliff limestone to the lead-bearing beds 

 beneath — unless, indeed, these lower beds should prove to be 

 beyond the sphere of action where the lead has been pro- 

 duced. This latter contingency is possible ; yet the richness 

 of the mines in the southern and western portion of the lead 

 district (at Apple river and Dubuque, for example), as com- 

 pared with some of the northern mines, seems to indicate that 

 the ore may still continue rich in the descending beds. Since, 

 however, this is, as yet, an unsolved problem ; and even if it 

 were solved, as it would require much capital to sink shafts 

 to the necessary depth, and since mines of this depth w^ould 

 doubtless be inundated w^ith water, and require steam-engines 

 to drain them, I have not considered it my duty to include 

 this southern portion of the district within the bounds of the 

 productive lead region ; although, hereafter, should the easily 

 accessible lodes be exhausted, and the demand for lead rapid- 

 ly increase, it may become so. 



" With regard to the magncsian limestone which underlies 

 the blue limestone and sandstone strata, and comes to the 

 surface of the extreme northeastern portion of the district, its 

 similarity in structure and composition to the cliff limestone, 

 including its disposition to form vertical fissures, and its pro- 

 bable identity with the rock formation in the Missouri lead 

 region, might induce the expectation that it, also, would be 

 rich in lead ore. It may be so ; but the frequent occurrence 

 of iron ore (brown oxide) in those townships where this for- 

 mation prevails* (as in the upper beds of the southern portion 

 of the district), together with the little success which has 

 hitherto attended the search after lead ore within its confines, 



* It is not improbable, from its similarity to the lead-bearing rock in 

 Missouri, that this lower magnesian limestone, if it be extensive north of 

 the Wisconsin river beyond the limits of our survey, may there yield pro- 

 ductive veins of lead ore. 



