48 ■ The American Geologist. J"'>-' ^'-^"i- 
clicniical and the fire resistin"; qualities have later been deter- 
mined in the laboratory they can then be distinguished as pure 
and impure, high grade and low grade, or ])0()rly refractory 
and highly refractor}-. 
While the present paper deals jirincipally with the blue 
clays of the Coal Measures, the term fire clay, of course, cannot 
properly be limited to these clays, as valuable refractory clays 
occur in strata both older and newer than the Coal Measures 
without any connection with coal seams of any kind. Thus, in 
Missouri, fire clays occur in Lower Carboniferous, Silurian, 
and Ordovician strata and in New Jersey in Cretaceous or 
more recent. Many of the residual kaolins of the Piedmont 
belt are highly refractory. 
The explanation commonly ofifered for the origin of the 
l)lue fire clavs of the Coal Measures is that they are the soils on 
which grew the vegetation that forms the coal seams lying on 
top of them, and the reducing and leaching action of the vege- 
table acids from the living and the decaying vegetation has 
changed the common clay to the refractory fire clay. This ap- 
pears to be a satisfactory explanation for many of the clays, 
but there are some of the deposits that are not satisfactorily ex- 
plained in this way. 
The deoxidizing and leaching action of both living and de- 
caying vegetation is illustrated in many of the bogs and 
swamps of the present, and in a less degree in the meadow and 
the forest soil. The organic acids of the vegetable matter are 
strong reducing agents and under their action the red and 
vellow ferric oxides are changed to the lower gray or bluish 
ferrous oxide in which form it is soluble in the acids, and 
where there is sufficient circulation the iron may be either car- 
ried awav in solution or segregated into beds of iron ore. 
Fragments of minerals containing alkalies and alkaline earths 
may be broken up by the vegetable acids into separate com- 
pounds and the alkaline substances dissolved in the water and 
part carried away in solution and part may be taken up by the 
plants and held in the carbonaceous matter of the muck oi the 
bog of the present or the coal bed of the older ]>eriod. It some- 
times happens that there is better drainage into the bog than 
out of it, and the iron leached from the surrounding region may 
be carried into the bog and deposited as an iron ore. 
