460 The Atmosphere Considered [Odtober, 
acid exhalations are very abundant at the surface of the 
earth, and are in great part ascribable to the oxidation or 
decay of organic matter which in the first instance derived 
its carbon from the atmosphere. 
The above case shows the result of slow decomposition 
at great depths ; but similar effects are induced by the decay 
of organic matter near or at the surface. In swampy 
grounds, lagoons, and deltas, such as those of the Missis- 
sippi and the Sunderbunds, the decay of organic matter must 
exercise a very powerful influence on the chemistry of the 
soils, rocks, and sediments with which the water charged 
with the compounds formed during the process of rotting 
comes in contact. Peroxides, such as those of iron and 
manganese, will be reduced to the proto state, and will be 
rendered soluble and carried away in solution, to be after a 
while re-oxidised and deposited in such masses as to be 
worth working as ores. Silicates of soda, lime, and mag- 
nesia will be decomposed, and removed as carbonates ; and 
sulphates, which are usually present in most waters, will be 
reduced first to sulphides, and eventually decomposed with 
evolution of sulphuretted hydrogen. Such a process as this 
may be observed every autumn in the North of Ireland 
during the maceration of the flax plant, which is placed in 
pits filled with water, and, being allowed to remain for some 
weeks, the softer tissues are rotted away, leaving the fibres 
fit for manufacture. The stench of sulphuretted hydrogen 
from the decomposing flax is almost unbearable. Having 
analysed the mud which subsides to the bottom of the flax- 
pits, I find that the reducing power of the rotting tissues 
are as described above. The clay in which the pits are sunk 
contains nearly all the iron present in the ferric condition 
when not subject to the aCtion of the plants, but in the mud 
from the bottom there are only proto-compounds, the iron 
mostly as carbonate. Nor is there a trace of peroxide of 
iron in the flax-water, but, on the contrary, plenty of ferrous 
iron. 
Clay-Ironstone . — After this fashion must have been formed 
the clay-ironstones of the coal-measures. The great 
swampy estuaries of that period may be regarded as 
gigantic flax-pits ; and the rotting vegetation not only 
altered other salts and compounds of iron to carbonates, 
but prevented the oxidation of such carbonate of iron as 
might have been carried down in solution, until in course 
of time it also was precipitated along with the clayey 
sediments. 
During such changes near the surface a very large 
