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332 DYNAMICAL GEOLOGY. [Book TIL. 
readiness to unite with any substance that can contain more of it, 
render oxidation a marked feature of the passage of rain over rocks. — 
A thin oxidized pellicle is formed on the surface, and this, if not at 
once washed off, is thickened from inside until a crust is formed over 
the stone. This process is simply a rusting of those ingredients 
which, like metallic iron, have no oxygen, or have not their full com- 
plement of it. The ferrous and manganous oxides so frequently found 
as constituents of minerals are specially liable to this change. In 
hornblende and augite, for example, one cause of weathering is the 
absorption of oxygen by the iron and the hydration of the resultant 
peroxide. Hence the yellow and brown sand into which rocks 
abounding in these minerals are apt to weather. 
2. Deoxidation.—Rain becomes a reducing agent by absorbing 
from the atmosphere and soil organic matter which, haying an 
affinity for oxygen, decomposes peroxides and reduces them to prot- 
oxides, This change is especially noticeable among iron oxides, 
as in the familiar white spots and veinings so common among red 
sandstones. ‘These rocks are stained red by ferric oxide (hematite), 
which, reduced by decaying organic matter to ferrous oxide, is usually 
removed in solution as an organic salt or carbonate. When the de- 
oxidation takes place round a fragment of plant or animal, it usually 
extends as a circular spot ; where water containing the organic matter 
permeates along a joint or other divisional plane, the decoloration 
follows that line. Another common effect of the presence of organic 
matter is the reduction of sulphates to the state of sulphides. Gypsum 
is thus decomposed into sulphide of calcium, which in water readily 
gives calcium carbonate and sulphuretted hydrogen, and the latter by 
oxidation leaves a deposit of sulphur. Hence from original beds of 
eypsum, layers of limestone and sulphur have been formed, as in 
Sicily and elsewhere (p. 64). 
3. Solution.—A. few minerals (halite, for example) are readily 
soluble in water without chemical change, and without the aid of 
any intermediate element. In the great majority of cases, however, 
the solution is effected through the medium of carbonic acid or 
other re-agent. A familiar illustration is the solution and removal 
of lime from the mortar of a bridge or vault, and the deposit 
of the material so removed in stalactites and stalagmites (p. 112). 
Another common example is seen in the rapid effacement of marble 
epitaphs in our churchyards. It has lately been shown that in the 
atmosphere of a large town with abundant coal-smoke and rain, in- 
scriptions on marble become illegible in half a century. Pfaff recently 
determined that a slab of Solenhofen limestone 2520 square milli- 
metres in superficies lost in two years by the solvent action of rain 
0180 gramme in weight, in three years 0°548, the original polish 
being replaced by a dull earthy surface on which fine cracks 
and incipient exfoliation began to appear. ‘Taking the specific 
gravity of the stone at 26, the yearly loss of surface amounts to 
1 
: aes 
The reducing action of organiv acids is further described in Section iii. 

