84 University Geological Survey of Kansas. 
within it. In such cases milling processes are difficult, as 
the solid rock is hard to crush, and must be ground toa 
comparatively fine powder in order to set free small particles 
of granules of ore so intimately mixed with the gangue. The 
so-called open ground and bouldery ground are places where 
secondary material is not abundant, but masses of primary 
flint of different size and shape may happen to occur here 
lying loosely together, having cavities filled with ore. On 
account of the absence of astrong cementing material the 
ground is open so that water can pass freely, and mine water 
is correspondingly troublesome, and extensive timbering is 
necessary to hold up the roof of the mine. ‘This condition is 
most characteristic of the border areas between heavy flint 
beds and limestone, so well represented at the Bonanza dig- 
gings along Short creek. : 
Similar results are found in places where primary flint is 
very badly fractured into small fragments with open spaces 
sufficient to permit accumulation of profitable ore bodies. 
In such cases ore is distributed in small granules through 
the open spaces and mining is conducted with considerable 
hazard on account of the loose condition of the roof, absence 
of cementing material being noted. Along upper portions of 
Cooper Hollow near the state line east of Galena are good 
illustrations of this kind. Here large bodies of primary flint 
were fractured to an excessive degree and zinc ore deposited 
in the fractures, so that almost every cubic yard carried from 
five to ten per cent. of blende. No sooner was an adit 
driven, however, than the roof began falling a little at a 
time, principally where the ground was the most open and 
water the most abundant. Practically no secondary chert 
exists here, and the ore itself is so easily broken that it serves 
as a poor cement. Less than a mile to the west, particularly 
on the ground of the South Side Mining Company, secondary 
chert is unusually abundant, with the result that the ground 
is excessively hard and timbering the mine unusually easy. 
Here large rooms are opened up from fifty to seventy-five 
feet across from wall to wall and the roof holds with perfect 
safety. In fact, large portions of the east part of Galena it- 
self are undermined without any apparent danger. 
