IOWA ACADEMY OP SCIENCES. 65 



The extensive zinc mine at Durango, five miles northwest of 

 Dabuque, has several points of special interest. The timber 

 range on which the diggings are located was once well known 

 as a large lead producer. The range has a width of 100 feet, 

 and is formed by three main crevices, with a general direction 

 S. 80° E. The openings occur ninety feet below the crown of 

 the hill, and where they are enlarged the three fissures unite 

 in caverns of immense size. In these openings the lead 

 occurred,. and above them, extending to the surface, the hill is 

 filled with zinc carbonate. The zinc is known to extend also 

 below the level of the lead. The mine is worked by means of 

 an open cut extending through the hill, with a width of forty 

 feet and a depth of about eighty feet. The crevices are more 

 or less open up to the surface. Several can be seen in the face 

 of the cut, and in them the ore is most abundant, though it 

 is also found mixed all through the fractured limestone. The 

 strata have been subjected to more or less strain, possibly 

 owing to the large caves below, and are broken into fragments. 

 The carbonate is found coating these pieces and filling the 

 spaces between, occurring also, as stated, in the open crevices. 

 The latter have a width of from one to two feet. In working 

 the mine the larger masses are blasted and the smaller ones 

 loosened with the pick. The ore is removed from the rock, the. 

 latter is carted off to the dump, and the dry bone, mixed with 

 more or less waste material, is carried to a neighboring stream. 

 Here it is washed by an ingenious contrivance which thoroughly 

 frees the ore from all sand and dirt. The method was invented, 

 by Mr. Goldthorp, superintendent of the mine, and is quite 

 extensively used about Dubuque. An Archimedes screw, turned 

 by horse power, revolves in a trough through which a stream 

 of water is kept flowing. As the screw revolves it gradually 

 works the ore up the gentle incline, while the water runs dowu 

 and carries with it all sand and dirt. Afterwards the dry bone 

 is picked over by hand and the rock fragments thus separated. 



During the past season eighteen men were employed at the 

 mine and the daily output was from fifteen to eighteen tons of 

 ore. This would mean a yield of over 2,500 tons for six 

 months, and is probably about the annual production of the 

 mine during the last few years. 



Most of tne zinc mines have been closed for nearly two years 

 on acount of the low price paid for the carbonate, the average 

 being only $5 to $6 per ton the past year. About 800 tons 

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