STATE GEOLOGIST. 61 
into slate or the clays may even be brought to the condition of an igne- 
ous rock as soon as the heat is sufficient to melt them. 
Steam especially in the superheated condition is a powerful agent 
in the decomposition of all silicates. This may readily be shown experi- 
mentally, when it will be found that even very acid silicates are decom- 
posed without trouble. When in Germany extensive attempts were made 
by Professor Meyer to burn lime at a lower temperature by means of 
superheated steam so as to produce a better product for the purposes of 
the sugar industry these were eminently successful as far as the lime 
burning was concerned, but had to be given up because the kiln linings 
of fire brick and the iron pipes were destroyed so rapidly. We can 
readily see that clays in contact with superheated stexzm will be com- 
pletely decomposed and their entire character destroyed. Similarly, the 
action of volcanic gases like carbon dioxide, hydrogen sulphide, etc., 
is bound to result in a material alteration. 
Chemical Action.—The chemical changes which clays undergo rep- 
resent a continuous chain of reactions which never cease. Though the 
_- chemical processes constantly taking place are infinitely numerous, we 
may distinguish principally two kinds of reactions, namely, oxida- 
tion and reduction. It is evident that these reactions involve prin- 
cipally those compounds which are easily oxidized and reduced, espe- 
cially the oxides of iron and manganese, though the latter is present in 
clays in such small quantities that it may be neglected. Ifa clay hap- 
pens to contain its iron in the state of the lower oxide, as the carbonate, 
or as the sulphide, FeS, (iron pyrites) the atmospheric oxygen brought 
in by water percolating through the deposit will convert it to ferric 
oxide or its equivalent, ferric hydroxide, or the sulphate. The whole 
character of the clay is thus changed, which is usually shown by the 
difference in color. A bank of shale, for instance, near the surface will 
possess a reddish or yellow color owing to the presence of the higher 
oxide of iron, while on working into it the color is found to be blue or 
gray, indicating ferrous iron. At the same time the surface shale will 
‘be found to be soft and easily worked while that in the hill is hard and 
tock-like. 
ConverSely, if an oxidized clay is exposed to reducing conditions 
resulting from organic matter the iron is changed to the ferrous condition, 
‘producing ferrous oxide and ferrous carbonate from the ferric oxide and 
ferrous sulphide from the sulphate. Hydrogen sulphide would naturally 
-convert the iron to the sulphide. 
There are a great many other possible reactions, such as the decom- 
position of ferrous or ferric sulphate and calcium carbonate resulting in 
the production of the ferrous or ferric hydroxide or oxide and calcium 
‘sulphate, carbon dioxide being evolved.’ Similarly, magnesium sulphate 
may be produced from the iron sulphides and sulphates as we know it to 
