118 G-eology of Sydney. 
immediately below there should be a bed of ironstone ? 
It will be remembered that one of the essentials of a 
good clay for porcelain is the absence of iron. Also 
one of the essentials of a good brick clay is a low per- 
centage of iron. It is only the clay that contains 
practically no iron that will burn white in a kiln. 
Brick clays, even when colourless, burn red. This is 
due to the fact that these clays contain iron as a 
protoxide , and this protoxide is converted on burning 
into the peroxide which gives the brick its red colour. 
It is then a matter of the highest importance, 
from an economic standpoint, that shales should be 
free from iron. And it is equally important that in 
places ironstone beds should be segregated, so to 
speak, rich enough in iron to render them fit for the 
extraction of the metal for manufacturing purposes. 
We will now follow the process by which iron- 
stones were formed in beds. I must, to make the 
matter clear for the general reader, premise that the 
following facts are familiar : — 
1. The iron in rocks is for the most part com- 
bined with oxygen. 
2. This gives rise to two oxides, the Protoxide and 
the Peroxide. 
3. The Protoxide is soluble in carbonated waters. 
4. Peroxide is insoluble in carbonated waters. 
5. The combinations of protoxide of iron are 
bluish, greenish, or colourless ; for example, 
brick clays. 
