316 GEOLOGY OF OHIO. 
blackband ores, clayband ores and “ flag” ores. They contain, as a rule, 
a smaller percentage of iron than the other ores of the series, but com- 
pensation for their poverty is made in whole or in part by their greater 
volume and also by the character of the foreign matter sometimes associ- 
ated with them. There is often enough carbonaceous matter in the seam 
to effect the calcination of the ore, and by the expulsion of the car- 
bonic acid of the ore and by the combustion of the organic matter 
of the seam, the proportion of metallic iron is raised from 25 to 50 
per cent. In volume, as compared with the other ores of the series, 
they may almost be said to give feet for inches, the maximum that they 
attain being 19 feet, and the working thickness of large areas rising to 
6 feet and over. 
The ores of this class are worked at three or more distinct hori- 
zons of the Lower Coal Measures of Ohio, and are of great economic 
importance. 
The second group includes those ores that owe their present forms 
to the obscure agency to which we give the name of concretionary 
force, a force which is allied to chemical force to this extent, that it 
gathers up and unites the previously scattered atoms of one or more 
chemical compounds. Of this group there are three distinct subdi- 
visions, which are named below: ) 
(a.) Kidney ores. 
(b.) Block ores. 
(c.) Limestone ores. 
The ores in which concretionary force is most distinctly shown are 
those known as Kidney Ores. They consist of masses of impure carbo- 
nate of iron, often rudely discoidal or ellipsoidal in form, and always 
bounded by curved surfaces. As a rule, they are composed of concen- 
tric layers or shells which are made very distinct by weathering. They 
sometimes have hollow cavities within, after weathering, and sometimes 
enclose masses of clay. Some of them, however, are crystalline at 
their centers, containing calcite or barite, or occasionally sulphide of 
zinc. They are generally quite close grained and heavy, when under 
good cover. They range in size from an inch to a foot in diameter. 
They are distributed in the beds of shale or fire-clay that make up so 
large and characteristic a portion of the coal measure strata, and from 
which their materials have been segregated. Sometimes they are gath- 
ered into distinct horizons, which the mincr can easily and economically 
