ORE-DEPOSITION IN TRUNK-CHANNELS 229 
that trunk-channels are lines of rapid ore-solution and removal 
rather than of ore-deposition. In reality the Ozark'Country is 
a region which is now being very rapidly depleted of its once 
rich ore-materials. Only when the main channels of groundwater 
flow become clogged in some way or other do ore-bodies now 
form in them. To this fact more than anything else is it due 
that ores are not everywhere evenly deposited in the vadose 
zone. It is likely that it is a fundamental law of ground- 
water movement that only where the movement is interrupted, or 
stagnation prevails, are ore-materials precipitated and localized. 
Still another feature has particular bearing upon this topic. 
Notwithstanding the fact that vadose waters generally tend not to 
drop their metallic loads in the rock-cavities through which they 
rapidly pass, mineral matter under these conditions and without 
regard to rate of motion may replace certain components in the 
wall-rock. This chemical interchange may take place along the 
walls of the channel or along the lines of the stratification-planes, 
jointing, or faulting. Whether mural or stratal in character 
there seem to be practical reasons for assuming that in the vadose 
zone cavity-filling and wall-replacement are partly, at least, direct 
functions of the rate of groundwater motion — the first prevail 
ing when the current movement is slow, and the second taking 
place mayhap usually when the groundwaters are rapid and free- 
flowing. 
The necessary consequences of a theoretical consideration of 
circulatory groundwaters have never been very critically com¬ 
pared with the recorded observations on the distribution of ore- 
materials in mines. Of course part of the waters moving hori¬ 
zontally in the vadose zone often issues at the surface of the 
ground as springs and becomes mingled with the surface-waters. 
At groundwater-level, or slightly below, the portion passing down¬ 
ward into the profound region at once drops a large proportion 
of its metallic salts to form the bonanza ore-zone. The part 
carried farther downward mainly enters into combinations which 
are not commonly ore-forming. Except at the top of the pro¬ 
found section of such channels few or no ore-bodies are deposited 
by these currents. When these downward-moving waters begin to 
ascend again, under ordinary circumstances, they are probably 
practically free from the common metals in solution; and on 
