40 University Geological Survey of Kansas. 
per.cent. However, it manifests a strong tendency to contain 
iron sulphide as an impurity. This impurity is present in 
such a varying degree that it greatly modifies the color of the 
blende. When pure, blende is almost colorless, or white, 
but with the presence of iron sulphide in the crystals the color 
gradually grows darker, passing through a series of yellow,. 
amber, brown and black. These colors produced in this way 
have given rise to such names as ‘‘rosin jack,’’ ‘‘amber 
jack,’’ ‘‘gray jack,’’ ‘‘steel jack,’’ and ‘‘black jack.’’ Usu- 
ally a fair estimate of the amount of iron present can be made 
from the color of the crystal, the color growing darker as the 
proportion of iron increases. This results in the production 
of many grades of ore entirely aside from the presence of 
pyrite or marcasite, ‘‘ mundic,’’ which may have been left in 
the ore from improper milling processes. Tor years ore buyers 
have been fixing the price of ore quite largely by the amount 
of iron present as shown by assay. With them it is imma- 
terial whether the iron is present as an impurity in the crys- 
tals of zinc blende itself, or a fragment of pyrite or marcasite 
which milling did not separate. The impurities are so 
abundant that by common consent sixty per cent. ore is taken 
as a standard. 
SMITHSONITE.— Zinc carbonate, ZnCO,. This is a valua- 
ble ore of zinc when found in sufficient quantity. When 
theoretically pure it contains 52.03 per cent. metallic zinc. It 
is a secondary ore always produced by weathering processes, 
oxidizing zinc blende into zinc sulphate, which is soluble. 
Ground water containing the zinc sulphate in solution comes 
in contact with limestone, or calcium carbonate, when a re- 
action takes place between the two, producing zinc carbonate 
and calcium sulphate. In this way occasionally zinc carbon- 
ate is produced in large quantities. However, it is almost 
unknown in the Kansas mines and never has been found in 
sufficient quantity to be of any considerable commercial im- 
portance. Smithsonite is more abundant in the zinc fields of 
northern Arkansas than anywhere else in the Mississippi 
valley. In Europe it is sufficiently abundant to be an im- 
portant ore. 
