APPENDIX—GEOLOGICAL SURVEY. 
441 
Such a theory is well adapted to explain the character of the 
sandstone of the lead district, and in but few places are the 
facts that support it better defined. If we follow along the 
line of the north and south axis already referred to, to the 
northern part of the state where these sandstones crop out, and 
the azoic formations become the surface rock, we shall find 
belts of iron ore crossing this axis at, or nearly at, right angles 
to it. The Penokee iron range is a good example. Coming 
south along this axis to where a thin layer of sandstone covers 
the azoic rocks, these belts of iron ore are seen in places pro¬ 
truding through the sandstone. The well known deposit of 
iron ore at Ironton, Sauk county, on the western extension of 
the Baraboo hills, is a good example of this. Still farther south 
where the sandstone has its full thickness, we find well defined 
belts of sandstone impregnated with iron ore in various forms. 
These impregnated belts conform to the bearing of the iron 
belts in the azoic, and cross the north and south axis in a very 
similar manner. The careful observer cannot fail to notice 
that there is a close relation between the belts of iron in the 
azoic formations and the impregnated belts of sandstone rest¬ 
ing on those formations; nor can there be but little doubt that 
the sandstone has been impregnated by solutions penetrating 
and rising through it from fissures connected with these iron 
deposits in the older formations. In speaking of the sandstone 
here, I include the two members of the series, the St. Peter’s 
and the Potsdam. 
Lower Magnesian Limestone.— Immediately below the 
St. Peters, and separating it from the Potsdam sandstone is a 
bed of limestone about 250 or 300 feet thick. It is known as 
the lower magnesian limestone, and is described in the report 
of 1862. I shall notice it here only in its relation to mineral 
veins. 
“This formation,” says Prof Whitney in his report, “is almost 
entirely made up, everywhere in the valley of the Mississippi; 
of an almost chemically pure dolomite, or a mixture of car¬ 
bonate of lime and carbonate of magnesia in the proportion of 
one atom, or equivalent of each.” Chemically it is the same as 
