62 ANNUAL REPORT 
enter the water from coal mines. Even free sulphuric acid may be pro- 
duced under favorable conditions. Hydrous silicic acid brought into a 
clay may be precipitated by calcium carbonate and iron compounds by the 
action of organic acids. 
All these reactions are coincident with the processes of solution, in- 
filtration and crystallization. 
If the tendency of the chemical reaction is to tales the basic constit- 
uents of a clay more soluble they are taken up by water, dissolved and 
carried somewhere else and allowed to crystallize there. A clay may thus 
become purified while at the same time another material is being charged 
with the elements removed. 
Solution and Recrystallization.—The matter of solution and crys- 
tallization has already been touched upon. Practical examples of this 
kind of action are quite frequent. Ina shale, high in sulphur compounds, 
these become oxidized and soluble in water and are removed by the water 
to a lower point where crystallization takes place. If from the ferrous 
sulphate caletuwm. sulphate is produced the latter will crystallize out as 
gypsum. The gypsum crystals thus: produced. may be frequently found 
in the soft clay at the outcrop of a shale deposit. Crystallization is. not: 
restricted to this one kind of substance, but may involve all of the soluble 
constituents and even may be observed in the clay itself where frequently 
the outlines of large crystals may be traced. 
The crystallization of soluble constituents in a clay often condemn it 
for building purposes since these salts give rise to the unsightly efflores- 
cence called “white wash” in the wall. 
Infiltration.— We have already seen how soluble constituents may be 
dissolved and taken away to another part of a clay bed. These 
soluble constituents may, of course, come also from another rock and may 
be precipitated in the clay. Thus, soluble silicic acid brought in by deep 
seated springs is readily precipitated in the clay by calcium carbonate, 
iron salts by various organic compounds. As a result we would find veins 
or streaks of the materials introduced by infiltration wherever the water 
flowed, as we frequently find clays showing iron stain in an irregular 
network. 
Leaching. —Wherever an impure but uniform deposit of clay is sub- 
jected to the action of organic acids and at the same time an outlet for the 
materials dissolved is provided it is bound to be purified by this leaching 
action. All of the soluble and also ordinarily insoluble compounds are 
made soluble and removed by the organic compounds. In time there will 
be left nothing but a mass closely resembling pure clay substance and 
whatever quartz was originally contained in the clay. Many of our Ohio 
fire clays have been produced in just this manner. 
Concretionary Action.—Concretionary action is often active in col- 
lecting materials like ferrous carbonate or calcium carbonate in the clay and 
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