HAwoRTH. | . Ore Deposits. 109 
find a mill doing custom work which does not produce con- 
centrates of both ores. As mining progresses year after year 
the ratio in tonnage between the two is gradually changing 
by the proportion of lead ore constantly decreasing. ‘This is 
partly due to a comparative shifting in prices. The price of 
zinc ore has increased gradually during the last ten years, 
while the price of lead ore has decreased, but not so much 
during this short time as during the decade between 1875 and 
1885. In the early days of mining at Joplin and Galena all 
leases were written on a basis of seven cents a pound for lead. 
Recently lead in the St. Louis market has averaged little more 
than four cents. This downward shifting of prices of lead ore 
and upward shifting of zinc ore prices have brought the two 
ores nearer together in value and have led mine operators to 
search for zinc ore as zealously as for lead ore. 
Lead ore and zinc ore are associated so intimately that in 
many cases a fragment the size of one’s hand contains both 
ores. Where lead ore occurs in large masses hand sorting may 
be resorted to profitably to separate the two ores. Even then 
hand sorting is only partially effective. But in most cases no 
attempt is made to separate the two by hand, the entire mass 
being sent to the mill and separated by machinery. 
It has already been pointed out in chapter V that the flint 
rock masses are full of fissures and even broken up to an ex- 
ceedingly great extent; that where limestone was present in 
a small amount it has been removed by the solvent action of 
water and flint and residual masses have fallen together in 
a very promiscuous manner. Ore bodies occur in the open- 
ings thus produced. Such openings are as varied in charac- 
ter and extent as the imagination can picture. Here little 
seams one hundredth of an inch in width are filled with ore, 
sometimes one kind, sometimes another; there a large open- 
ing a number of feet in extent is filled with water and mud 
and ore with the greatest diversity of conditions. Where 
limestone was absent the fissures streak the flint in every 
imaginable manner and ores are scattered through the entire 
mass in small quantities, filling such fissures and acting as 
a cement which weakly holds the fragments together. The 
secondary flint is present in fissures and openings in a man- 
