IOWA \CADEMY-OF SC1E^XES. 49 



first mine to be worked for zinc was the McNulty, often called the "Avenue 

 Top" mine, at the head of Jiilien avenue. Dubuque. This had previously 

 been operated for lead and $25,000 worth is said to have been taken from it. 

 The galena gave out in the crevice and a short distance beyond the zinc car- 

 bonate began to appear. It is estimated that this mine has yielded not less 

 than $50,000 worth of the latter. After the sale of the first dry bone many 

 began at once to search for it and numerous mines were soon being oper- 

 ated. Old lead mines that had been abandoned were again opened and 

 worked for zinc when the associated ore began to appear. 



A slight examination of the great mining regions of the globe will show 

 that they are situated in regions of disturbance in the earth's crust. The 

 strata have been more or less tilted from their original horizontal position, 

 or are fractured and igneous masses intruded into them. lu other words, 

 the ore deposits of the globe conform to the general law stated by Humboldt 

 that "the deposits of the precious metals and of lead, zinc, and mercux'y 

 are usually associated with intrusions of igneous rocks." 



The zinc deposits of the Upper Mississippi form a notable exception to 

 the above law. They occur in practically undisturbed strata which show 

 no evidence of having been subjected to metamorphic agencies or of having 

 any connection with igneous masses. 



The manner in which these deposits occur is also unusual. They are 

 not in true veius, tilling fissures produced by some deep seated cause and 

 extending to a considerable depth, but the zinc is found in crevices which 

 have a comparati\iely limited extent downward, and show no evidence of 

 having been connected with igneous masses below. 



Whitney says, in connection with the Upper Mississippi region,* "These 

 deposits approach most nearly in character to what have been designated 

 as gash veins; but they are in some respects peculiar in character, no min- 

 ing region exactly resembling this in mode of occurrence of its ores, having 

 been observed by me in any part of the world, unless it be in the Missouri 

 mines in which the conditions of the Upper Mississippi region are closely 

 imitated although upon a somewhat limited scale." In Missouri the zinc 

 ores occur in the sub-Carboniferous formation. There is in that region an 

 apparent connection between the surface drainage of the country and the 

 deposits. 



Other occurrences for zinc are those of Kansas, New Jersey, Pennsyl- 

 vania, Tennessee and Virginia. The principal foreign countries for the 

 production of this ore are the Rhine District and Belgium, Silesia, Great 

 Britain, France and Spain, The fii'st named region has for years yielded 

 more than the other four combined. 



The zinc ore of Iowa is found in crevices in the Galena limestone. The 

 strata of that region are cut by fissures, and it is in the expansions or open- 

 ings of these that the deposits occur. There is a very noticeable uniformity 

 in the direction of these crevices. With few exceptions they have either an 

 east and west or north and south direction, the former being much the more 

 common. Besides these two sets there are others, known as "quarterings," 

 that cross the main ones at varying angles. The larger crevices and those 

 carrying most of the deposits are the east and wests, while the north and 

 souths are narrow, and, when occurring in them, the ore is in sheets. The 



* Wlacousia Report, vol. 1, 1862. 

 4 



