IOWA ACADEMY OP SCIENCES. 
65 
The extensive zinc mine at Durango, five miles northwest of 
Dubuque, 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 fiowing. As the screw revolves it gradually 
works the ore up the gentle incline, while the water runs down 
* 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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