50 IOWA ACADEMY OF SCIENCES. 



latter set in many cases serve as feeders to the major clefts, and at their 

 intersection large bodies of ore are apt to occur. 



The zinc as a rule is found in what are called "openings." These are 

 formed by the widening of the crevices due to decomposition or solution of 

 the rock in these particular layers. These cave-like expansions usually 

 include a number of strata whicii form more or less irregular walls of either 

 side. At the surface the fissui'es commonly appear simply as a seam in the 

 rock, which followed down probably contains little or no mineral until it 

 suddenly widens out into the opening where the ore, if any, will be found to 

 occur. The dimensions of these openings are very variable, their height 

 being all the way from three or four to forty feet, and their width from one 

 or two up to twenty and in a few instances even forty feet. They are com- 

 monly limited above by a hard persistent layer of limestone appropriately 

 called by the miners the "cap-rock." The latter is almost invariably cut 

 through by a seam which may be so small as scarcely to be distinguished, or 

 by an open fissure of varying width that often cai-ries ore. The opening 

 frequently extends up above the main level of its roof, forming large cone- 

 shaped or irregular cavities or "chimneys" as they are called. On the other 

 hand it may widen out and form large rooms or "caves" filled with the zinc 

 ore. 



It is not uncommon for the ore-beai'ing cavity to be divided or almost 

 blocked up by a large mass of limestone known as the " key-rock." This 

 obstruction has probably been left because of its greater compactness, that 

 has enabled it to i-esist the destructive forces that have removed the sur- 

 rounding rock. 



The expanded crevices often contain rounded blocks called "tumblers," 

 that, like the key-rock, have escaped decomposition, their edges and cor- 

 ners worn away by air and water. 



The term "opening" is liable to be misleading as conveying the idea of 

 an open space. They are, as a matter of fact, usually tilled with ore mixed 

 with more or less clay and rock fragments. Even where large caves are 

 foi'med these may be tilled to the top with crevice material mixed with zinc 

 carbonate. On the other hand openings are found empty or only partially 

 filled with clay, and can be traversed for hundreds and even thousands of 

 feet through passages where no work has been done to clear the way. 



The ore-bearing crevices, when followed down, are found to widen out 

 into several openings, one below the other. The upper one is called the 

 "first" opening, the next below is the " blue rock" opening of the miners, 

 and still lower is a third and fourth. In the mines of the Dubuque region, 

 the first is the only one that has been largely operated, the water hindering 

 progress at the lower levels. The second has, however, been woi-ked when 

 possible. In the Center Grove mines, two miles west of Dubuque, ore has 

 been removed from the thii'd and, in one case, from the fourth opening. 



The ores of zinc found in the Iowa mines are the carbonate (Smith- 

 sonite), sulphide (Sphalerite), and, in comparatively small amounts,' the sili- 

 cate (Calamine). 



The carbonate or dry bone is by far the most common. It occurs in a 

 great variety of forms; in cellular masses; as botryoidal coatings; in earthy 

 masses and impregnating the rock. It is found coating galena crystals and 

 also entirely I'eplacing the lead and forming pseudomorphs. Several inter- 



