The Iron Ores of the Mesabi Range. — Spurr. 339 
scant oxidation, waters deposit silica and remove iron in 
solution. Thus the portions which are most porous become 
richer in iron and lose their silica ; this iron is derived mainly 
from the adjacent unoxidized parts of the rock, and into 
these unoxidized parts the silica from the oxidized portions 
is constantly carried. The final result is an almost complete 
separation of iron and silica. 
Chemistry ok the Process. 
The chemistry of this process seems to be approximately 
as follows : 
Most atmospheric waters contain, among other things, car- 
bonic acid, oxygen, and certain amounts of the alkaline salts. 
Carbonic acid gives water the power of taking iron into solu- 
tion : while the alkaline salts are slower but ready solvents 
of silica, especially in the finely divided cryptocrystalline or 
chalcedonic state in which it is usually found in these rocks. 
These waters find their way into the rock, and to some degree 
into the firmest portions. As they begin to penetrate inward 
from the crevice along which they came, the oxygen unites 
with the unoxidized elements of the rock, decomposing some 
of the minerals, and extending the weathered zone. By the 
decomposing action of this oxygen, carbonic acid ma} T be set 
free; and the same product may result from the action of 
acids, either brought in by the water or developed during de- 
composition. The water thus passes further into the rock 
free from oxygen, but highly charged with carbonic acid. 
Aided by the pressure under which it is placed, it readily 
dissolves tile iron. The affinity of the alkaline solvents for 
the silica is so much weaker than that of the carbonic acid 
for the iron that the water becomes saturated with iron, with- 
out any appreciable quantity of silica being taken up. The 
solution finally finds its way again out into a zone of greater 
oxidation, sucli as a crack. Upon the access of oxygen and 
the release of pressure, most of the iron is precipitated. As 
the solvent power of the water i< now renewed, the alkaline 
salts become able to take siliea into solution. Thus iron re- 
places silica. Next the water again finds its way into an un- 
oxidized region, and under these conditions again takes iron 
into solution, which compels tin- precipitation of some of the 
silica; so that here silica replaces iron. Finally, this process 
