4?o 
[October, 
The Atmosphere Considered 
and the water of combination was doubtless afterwards given 
off spontaneously, in the same way as by hydrate of alumina 
and the hydrated forms of silica. There is indeed consider- 
able analogy between the hematites and the colloid forms of 
quartz. It is only necessary to compare these pisolitic and 
botryoidai iron-ores with the calcedonys to see this, and the 
comparison would be in favour of the aqueous origin of such 
iron-ores were fresh proof needed. 
It will be obvious that the reactions sketched out above 
with regard to iron ores and compounds applies equally to 
all other minerals capable of being oxidised or reduced. 
Copper pyrites, for instance, is often oxidised to sulphate, 
and the carbonate altered to oxide just in the same manner. 
Antagonistic Action of Carbonic Axdd and Oxygen . 
Clearly, then, the carbon and oxygen derived from the 
atmosphere sustain antagonistic parts in their adtion on 
rocks and minerals. They are perpetually warring the one 
against the other, and thus keeping up a circulation between 
the earth and the air. The carbon reduces the oxides when- 
ever it encounters them, and the oxygen replaces the car- 
bonic acid of carbonates with the same inveteracy. The 
combined effects of these elements in geological transforma- 
tions is extraordinary when we come to reflect' on it. 
Regarded from an utilitarian point of view, to them we owe 
probably every metalliferous deposit of value in the world. 
I have shown how a highly ferruginous rock, such as basalt, 
containing proto-salts of iron, which are soluble in carbonic 
acid, might be added on diredtly by that acid from the atmo- 
sphere. But there are cases where insoluble compounds of 
iron in small quantity, locked up in rocks, are, by the reducing 
adtion of the carbon of decaying vegetation, liberated, and 
finally accumulated in such quantities as to be of commer- 
cial value. Soils and clays contain small portions of per- 
oxide of iron, which is insoluble. The decay of vegetation 
or other organic matter robs this of oxygen, giving rise to 
carbonic acid. The resulting protoxide is soluble in water 
containing carbonic acid, or other organic acids, and is 
carried down into lakes or fissures, where, again absorbing 
oxygen, it forms beds or veins of hematite. 
While insoluble oxides are rendered soluble and allowed 
to accumulate in this way, soluble sulphates are reduced to 
insoluble sulphides, — iron pyrites, copper pyrites, zinc 
blende, galena, &c., — and, as Sterry Hunt puts it, “removed 
from the terrestrial circulation,” for a time at least. Such 
