ransome.J THE GROUND WATER. 141 
has actually been effective. Both in this mine and in the Silver Lake 
mine it is stated by those in charge that rich bunches of ore are com- 
monly associated with secondary or "post-mineral" Assuring, which, 
if true, is significant of some secondary enrichment below the zone of 
oxidation. 
THE GROUND WATER. 
In many mining regions the level at which the ground water stands, 
or, more accurately, the shape of its surface and the relation of the 
latter to the topography, is a matter of much importance, as by it are 
frequently determined questions of mine drainage and the depths at 
which oxidized ore changes to unoxidized ore. In the Silverton region, 
however, erosion has proceeded with such celerity relative to processes 
of oxidation and weathering that the change in question is largely 
independent of the depth of the ground water, and the latter is 
accordingly of less moment to the miner. 
In most regions of considerable relief the upper surface of the 
ground water corresponds to a very subdual copy of the topographic 
relief. In the exceedingly rugged and much fissured Silverton area, 
however, the ground-water surface is very much loss accentuated than 
the topography. Not only is the water tapped and drained away by 
the deeply cut ravines, but its level is often modified by the tunnels 
run into the mountains, sometimes several thousand feet below the 
crests of the ridges. Under these conditions it is often practically 
impossible to predict at what depth the ground-water surface will be 
encountered in mining operations. 
Tunnels driven in from the floors of t he high basin do not, as a rule, 
encounter permanent ground water. Those run below the basins, 
such as the Unity tunnel, or from the bottoms of the deeper ravines, 
such as the Highland Mary tunnel in Cunningham Gulch, the Bonanza 
tunnel near Animas Forks, the Old Lout tunnel in Poughkeepsie 
Gulch, the Revenue tunnel, t lie Noil h Star t unnel on Sultan Mountain, 
and the Empire tunnel near Silverton, usually tap the ground water 
and artificially lower its surface within the overlying rock masses. 
In the shafts of the Red Mountain district permanent water is encoun- 
tered at depths varying from 50 to 200 feet belowthe surface, depend- 
ent upon the location of the shaft and the seasonal fluctuations of the 
ground-water surface, which sometimes exceed 50 feet. In the Red 
Mountain district the ground-water surface is nearest the topographic 
surface in spring or early summer. 
REPLACEMENT DEPOSITS. 
Between certain replacement deposits and the metasomatic impreg- 
nations frequently accompanying fissure veins or ore stocks no sharp 
distinction can be made. Whether the deposit shall be classed as a 
vein or as a replacement deposit may depend simply upon the quan- 
titative relation between the ore found within the walls of the fissure 
