THE CLAYS. 647 
removed from the washing machines used in purifying clays finds ready 
sale to the flint mills for making potters’ silica. 
Feldspar and mica are found in very many clays, parti. ularly in those 
which have never been transported far from the parent rock. In clays 
which have been much disturbed before settling to the beds in which we 
find them, mica is not likely to remain in large amount. Feldspar acts as 
so much sand, except in its tendency to kaolinize whenever conditions 
favor. 
Since both these minerals are sources of potash and the alkalies, 
it is highly probable that they are the source of these bodies in clays ; it 
is certain that. they frequently are. It is rather hard to picture the free 
hydrate or even the soluble silicates of potash existing in bodies which 
owe their present structure and position to slow deposition in water. 
The hydrated sesquioxide of iron is common to all clays in greater 
or less extent. Even in the whitest of china clays some iron is always 
obtained. A recent example in the laboratory of the State University 
showed more than one per cent. in a silicious clay of snowy whiteness, 
though that amount is usually sufficient to color a clay to a buff or yellow 
tint. Ironis the great coloring agent in clays, and makes all tints from 
lightest buff to cherry red. It is probable that ferrous oxide and sili- 
cates of iron exist in clay, in which state they give it a blue or gray 
tint. Iron is found in all amounts from traces up to seven or eight per 
cent., or possibly more. The red brick and tile clays of the drift fre- 
quently run over five per cent. 
Lime and magnesia are found in small and persistent amounts in 
nearly all clays. In the clays from which the famous Milwaukee cream- 
colored bricks are made, the proportion runs up to twenty-three per cent. 
carbonate of lime, and seventeen per cent. carbonate of magnesia, with 
nearly five per cent. of iron. The average brick clays of the drift show 
from three to ten per cent. of lime, and in these uses it is a valuable agent, 
but it would be quite fatal to any of the higher uses of clay. Thealkalies, 
i. €., potash, soda and lithia, are found in all clays in greater or less extent, 
though not all together by any means. Potash is most common and most 
detrimental, lithia is most infrequent and in the smallest amounts. Its 
presence has not heretofore been noticed as an element in Ohio clays, 
but once detected, it was found in a number of samples. Mention has 
been made of mica and feldspar as the probable sources of the alkalies 
in clays, and this theory is strengthened from the fact that the largest 
