Some of the crevice and residual deposits worked by the early miners cropped out at the surface, but most of them were covered by earth. Other crevices were exposed in the bluffs of the Mississippi River and extended back into them for 1,000 feet or more. [Illustration: Figure 6—Diagrammatic cross section of two crevice deposits. A reaches ground surface and is filled with clay; B is only partly clay filled. Galena coats parts of the walls and occurs as pieces scattered through the clay. Typical crevices are about 3 feet wide and 5 feet high.] SOIL A GALENA CLAY AND ROTTED DOLOMITE B GALENA CLAY AND ROTTED DOLOMITE DOLOMITE When the richer deposits of ore in the crevices were worked out, some mines were deepened into the dolomite bedrock, but usually less rather than more galena was found. As the amount of galena decreased, however, another mineral, which had been present before in only small amounts, was found in increasing quantities. This was sphalerite—a yellow, brown, or black mineral of resinous appearance that is composed of zinc and sulfur (ZnS). It does not look like a metallic ore. At first the sphalerite was not used because there were no smelters in the area that could separate the zinc from the sulfur with which it was combined in the ore. Between 1850 and 1870, however, smelters were built in northwestern Illinois and southern Wisconsin and the ore was shipped there. Sphalerite is now the principal ore mineral produced in northwestern Illinois. The sphalerite and galena of southern Illinois, described later, are similar to that found in northwestern Illinois. [Illustration: Figure 7—Crevice lead mine. Miner pries ore loose and pushes it into car at bottom of crevice. Shiny galena layer occurs above his head and another at level of his waist.] _Mining and Milling._—One large mine is producing zinc and lead near Galena at present. Smaller mines operate irregularly. The ore may occur as pockets, irregularly shaped rather flat masses, vertical or inclined veins (fig. 8), or small particles scattered through the dolomite. The principal ore bodies that have been worked in recent years have been of irregular shape, both horizontally and vertically, and usually have been between 50 and 200 feet wide and from a few to as much as 100 feet high. They lie at a depth of roughly 300 feet. Blasting is required to loosen and break the ore. At the large mine the ore is brought to the surface by a hoist. In some relatively shallow mines an inclined tunnel has been driven to the ore body and the ore brought to the surface in trucks. [Illustration: Figure 8—Diagrammatic cross section of a zinc ore deposit showing flats and pitches. The deposit occurs in limestone and dolomite. Much of the ore is a mixture of dolomite, limestone, calcite, and sphalerite. Typical dimensions of such a deposit are about 30 feet high and 100 to 200 feet wide.]