MINING GEOLOGY 
339 
primary deposition and reworking of the iron resulted in the 
production of magnetite. Precipitation of this mineral in water 
at ordinary temperatures is not a reaction commonly observed in 
nature. There is reason to believe that the precipitate was a chert, 
with ferrous silica, and ferrous carbonate and more or less 
limonite. Oxidation and formation of intraformational conglom¬ 
erate probably produced limonite. Change from these minerals to 
magnetite i^ a sort of transformation that may take place in 
several ways. Ferrous iron of the silicate and carbonate and of 
the more oxidized limonite reacts directly to form magnetite. If 
the oxidation be very complete a reducing agent is readily avail¬ 
able in the organic matter which must have been present in 
certain beds. These and other reactions have been sufficient, 
during the long ages of recrystallization under conditions of heat 
and great pressure, to transform most of the iron oxide into the 
mineralogical form of magnetite. This process is not touched 
upon by either Van Hise or Leith. 
In summation, deposition probably occurred in shallow water 
by precipitation, mainly as an organic process, resulting in lean 
ferruginous cherts, with more or less siderite, ferric oxide, and 
greenalite. Alternating with periods of precipitation came periods 
of solution, leaching, oxidation and wave-action, producing intra¬ 
formational conglomerates, granular rocks much richer in iron, 
and doubtless some layers of pure ferric oxide. 
The richer deposits of magnetite are so characteristically in 
the granule and conglomerate zones that it is possible that the 
primary leaching was a determining factor in the development of 
the richer magnetites from the lean ferruginous cherts, greenalite 
rock, etc. Later, when covered by other beds there may have 
been more or less concretionary rearrangement. Deep burial 
under the slates developed heat and pressure which recrystallized 
much of the formation. The iron minerals reacted at this time 
with one another and with organic matter, and probably with 
other reducing or oxidizing agents, thus producing the magnetic 
oxides of iron. Recrystallization produced shrinkage cracks, and 
regional tectonic movements developed folds but appear to have 
had no appreciable effects on the concentration of the iron. 
Grout. 
