HAYES AND 
ECKEL. 
] OCHER DEPOSITS IN CARTERSVILLE DISTRICT, GA. 429 
only hydrated ferric oxide, a small amount of alumina, and a trace 
of manganese oxide, the latter giving it a slightly greenish tint. 
Some portions of the Cambrian quartzite contain interbedded sili- 
ceous shales, and the silica in these has also been replaced to some 
extent by iron oxide, producing an ocher which is inferior to that 
derived from the quartzite, since it contains considerable clay — prac- 
tically all the argillaceous matter originally contained in the shales. 
Embedded in this ore are numerous small cubes of pyrites, or rather 
limonite pseudomorphs after pyrite. These were probably an origi- 
nal constituent of the shales, before the replacement occurred. 
The above-described structure of the ocher and the inclosing quartz- 
ite, particularly as observed under the microscope, throws consider- 
able light upon its mode of origin. The forms of the residual sand 
grains in the ocher and of the siliceous skeleton about its border 
were evidently produced by solution. It seems probable, therefore, 
that the iron oxide is a direct replacement of silica. The faulting of 
the region, by fracturing the rocks, afforded favorable conditions for 
the percolation of surface waters to great depths; and since the fault- 
ing was doubtless accompanied by the development of considerable 
heat, the region was probably characterized by numerous thermal 
springs. The work of Van Hise and others has shown that, under 
favorable conditions, especially under great pressure and at high 
temperatures, silica becomes one of the readily soluble rock constitu- 
ents. It appears that, under certain conditions, a carbonic acid solu- 
tion of iron carbonate, meeting an oxidizing solution, precipitates its 
iron as hydrated ferric oxide and at the same time dissolves silica. 
The conditions for this reaction seem to have been present in the 
Cartersville region. Water, containing in solution iron carbonate or 
other ferrous salts derived from the decay of surface rocks, must have 
penetrated to considerable depth, particularly through the shattered 
quartzite. But in addition to this solution of iron percolating down- 
ward from the surface, the open fissures probably afforded abundant 
opportunity for the free circulation of water containing oxygen. The 
two solutions coming in contact, the iron carbonate was oxidized and 
precipitated as limonite, in the place of silica dissolved at the same 
time. The solution of the silica, which is the part of the process diffi- 
cult to understand, may have been assisted by the presence of alkalies 
in the oxidizing solution. It was probably greatly assisted by the 
heat which must have resulted from the faulting. It is also possible 
that carbonic acid, in the so-called nascent state, at the point where it 
is freed from one compound, may be a much more efficient solvent for 
silica than in its ordinary condition. 
Numerous open passages and cavities penetrating the quartzite and 
the bodies of ocher are met in mining. The smaller cavities are gen- 
erally lined with a crust of small quartz crystals, while the larger ones 
frequently contain beautiful crystals of barite, which were probably 
