50 GEOLOGY OF ILLINOIS. 



though widespread in its occurrence, no heavy bodies of mineral are 

 found as float mineral. This form of mineral deposit results from the 

 decomposition of the overlying Galena limestone, and in many cases it 

 has settled down almost in the exact spot where the rock containing it 

 once existed. 



The mineral in the rocks occurs in what is known as " gash veins," 

 and takes the forms of cog, dice, chunk, sheet, float, or fibrous mineral, 

 as modified by circumstances. The predominant forms of deposit are 

 the vertical crevices, and their modifications into the flat sheet and flat 

 sheet openings. A crevice is a perpendicular or nearly perpendicular 

 opening in the rocks, of varying width and depth. When filled with 

 galena, the deposit is called " sheet mineral." The sheet varies in thick- 

 ness, from a mere seam the thickness of a knife blade, up to three inches 

 or more in thickness. The vertical crevices have a certain well-marked 

 parallelism to each other, and an approximate north and south and east 

 and west direction. The east and west are, by far, the most fully devel- 

 oped, and contain, by far, the largest deposits of mineral. These 

 crevices are known by the various names of " leads," " lodes," " cracks,'? 

 "veins," "ranges," and "diggings." The predominant form of mining 

 in this county is that of the working of the vertical crevices. These 

 are, by far, the most productive, and are characteristic of the upper and 

 middle of the Galena limestone. The modifications of the vertical 

 crevice are the crevice opening, pocket opening, chimney opening, and 

 cave opening. They are all produced by the same causes. The crevice 

 opening is an expansion of the crevice to the width of several feet in 

 some instances ; the cavity is often filled with red ocher and ferruginous 

 clays, intermixed with loose stones and heavy masses of galena. The 

 pocket openings are a succession of irregularly-shaped small openings 

 in the crevices ; the chimney opening is a rather large expansion of the 

 crevice, extending upwards to a point resembling a chimney and the 

 cave opening is a large crevice opening, widening out into cave-like pro- 

 portions, floored often with stratified clays. In these openings the 

 galena is found lying over the bottom, mixed with the materials with 

 which they are filled, crystalized in blocks or cubes over the walls, and 

 hanging pendant from the roof. Some of the masses of mineral weigh 

 thousands of pounds, and it is said one mass was found in the mines of 

 Captain HARRIS, weighing half a million of pounds, and worth thirty -five 

 thousand dollars. 



These various openings are caused by the decay or disintegration of 

 the rock on the sides of the crevices, owing to chemical agencies work- 

 ing round the mineral deposits. If the dirt remains where it was formed, 

 the mineral and nodular masses of the rock will be found imbedded in 

 it j sometimes the dirt has been removed and the lead alone remains. 



