Mineral Deposits of Southwest Wisconsin. — Blake. 239 
Area and Geology. 
The lead- and zinc-ore region of Wisconsin extends through 
portions of Grant, Lafayette, and Iowa counties, and has an 
area of about 1,776 square miles. It lies just north of the 
Illinois state-line and south of the Wisconsin river, extending 
in a general north-easterly direction from the Mississippi. 
This is also the general direction of the groups of deposits 
and of the main lines of ore-occurrence. 
The geology is simple. The ore-deposits are confined to 
nearly horizontal strata of dolomite and limestone of Lower 
Silurian age, which lie between the Potsdam sandstone and 
the equivalents of the Cincinnati or Hudson .River shales. 
These strata, enumerated from below upwards, include the 
Lower Magnesian limestone, the St. Peter's sandstone, the 
Trenton limestone, and the Galena limestone,* or dolomite, 
this last having a vertical thickness of from 125 to 200 feet, 
and being the galena-bearing rock. It rests upon the beds of 
the Trenton limestone, a compact blue limestone generally 
known in the lead- and zinc-region as "glass-rock," from its 
brittleness and conchoidal fracture. The two formations are 
separated in the neighborhood of Shullsburg and New Dig- 
gings, in Lafayette county, by thin layers of a brown bitumi- 
nous shale called " oil-rock," upon which, in that vicinity, the 
zinc-deposits spread out and end, though in other districts 
they are known to extend lower into the blue limestone. 
The oil-rock, so far as yet known, attains its greatest de- 
velopment in the mines in the vicinity of Shullsburg \ and 
along the Shullsburg branch, notably at the Dry-Bone dig- 
gings and at the Galena Level, where it is overlaid by a strong 
sheet of sphalerite and barite with crystallized galenite. 
*The Cliff limestone of Owen (1840), known also as the Upper Magne- 
sian limestone, but including other formations as high in the geologic 
series as the Helderberg limestones of the Devonian. The area of the 
Galena limestone, which may be taken as limiting the occurrence of 
lead-ore and also zinc-ore, is given by Strong (Qeol. Wis., n., p. 681) at 
969 square miles. 
t Notably in some mines of the Hempstead; at the Butler shaft on 
the Little Giant tract; and at several other places on the property of 
the Wisconsin Lead and Zinc Co. At the Galena Level it underlies an 
extensive sheet of jack (blende), and is interstratifled therewith. The 
oil-rock, when dry, burns with a bright flame and much smoke, and it 
yields upon distillation a thick, viscid petroleum. 
