THE LOWER SILURIAN ROCKS. 547 



points several miles apart, this sandstone may be seen verj highly 

 charged with the brown iron oxide, which, at times, almost entirely 

 excludes the sandstone, having then mingled with it much of the red 

 or anhydrous oxido. It appears quite probable that the amount of 

 merchantable ore to be obtained in this neighborhood is sufficient to 

 warrant exploitation. Yery ferruginous sandstone, at a similar ho- 

 rizon, occurs at other points along the Baraboo ranges, but nowhere 

 else have promising indications been observed. 



THE LOWER MAGNESIAN LIMESTONE. 



Succeeding the uppermost layer of the Potsdam series, the Madi- 

 son sandstone, is a very persistent and wide spread bed of magne- 

 sian limestone, to which Owen gave the name of Lower Magnesian, 

 to distinguish it from another equally persistent, and in many respects 

 quite similar, magnesian limestone, that occurs higher in the series. 

 To the latter the name Galena limestone has since become attached, 

 whilst no other designation has been given to the lower formation. 

 In neither case can the term magnesian be regarded as at all dis- 

 tinctive, since almost all of the limestone beds of Wisconsin, including 

 the whole of the great thickness belonging to the Niagara group, are 

 highly magnesian, the only exception to the general rule being the 

 Blue limestone of the Trenton group. There appears to be but little 

 doubt that the Lower Magnesian is nearly the exact equivalent of 

 the Calciforous Sandrockof New York and Canada, with which form- 

 ation, indeed, it is nearly continuous through the northern peninsula 

 of Michigan and Canada West. 



The surface extent of the Lower Magnesian limestone in Central 

 Wisconsin is not nearly so great as that of the Lower sandstone, being, 

 in all, not more than about 75 square miles. The main belt occupied 

 by the formation enters Columbia county on the northeast ,with a 

 width of about 6 miles, and, spreading further and further west as it 

 is followed southward, occupies much, or all, of the towns of Eandolph, 

 Scott, Springvale, Courtland, Lowville, -Otsego, Fountain Prairie, 

 Columbus, Hampden, Leeds, Arlington, Lodi, and West Point in 

 Columbia county, and large portions of Roxbury, Berry, Dane, 

 Springfield, Yienna, Westport, Windsor, Bristol and York, in Dane 

 county. Still further south, again, the formation occurs only in nar- 

 row areas, crowning the summits of the ridges between the valleys in 

 which run the several head streams of the Catfish river, or forming 

 narrow strips between the low ground of the valley of that stream, 

 and the higher country wliich on each side is occupied by the St. 



