114 GEOLOGY OF OHIO. 
twelve workable seams of coal. As a consequence of this arrange- 
ment, the amount of coal underlying any given county or town in the 
coal area depends on its proximity to the center and deepest portion of 
the basin. So that while we have an aggregate of about 12,000 square 
miles of territory underlain by coal, not all parts of this are equally en- 
dowed with this great source of wealth. Along the margin of the coal 
basin, in many places, only a single coal seam is present, but the peculiar 
excellence of this one compensates in part for the deficiency in quantity. 
The aggregate thickness of all the beds included in the section of the 
south-eastern and deepest portion of our coal basin is perhaps fifty feet. 
The average coal contents of our territory may, therefore, be taken as 
something like the mean between the minimum, a single seam four to 
five feet, and the maximum reported above, or, in other words, twenty-five 
to thirty feet of workable coal. 
The coal seams which give character and value to the formation that 
includes them compose, therefore, but a small portion of the mass of 
strata with which they are associated. The other elements in the sec- 
tion are sandstones, shales, limestones, fire-clay, and iron ore. The na- 
ture of the materials forming the Coal Measures, their relations and 
relative quantities, will be best learned from an inspection of the en- 
graved section of our Coal Measures which accompanies this chapter. 
By referring to this section and the many others published in our 
reports, it will be seen that the elements composing the Coal Measures 
occur in an order of superposition that is so constant, or at least so fre- 
quently repeated, that it cannot be a matter of chance, but must be the 
expression of a general law. The order of sequence to which I have 
referred, and which will be noticed in these sections, is this, namely, 
that the coal strata almost invariably rest upon beds of fire-clay. They 
are also almost always covered with shale of greater or less thickness, 
and this in turn is overlaid sometimes with a sandstone, more rarely 
with limestone; and thus each section is susceptible of division into 
series of three or more members each, in which the elements hold nearly 
a constant relation to each other. These strata will be considered in 
the order of their occurrence, and as far as possible the history of their 
formation will be deduced from the facts which they present. For sev- 
eral reasons it is most natural and convenient to consider the fire-clays 
as forming the base of each series. In all ordinary circumstances, these 
are continuous sheets from one to twenty—generally three to four—feet 
thick, of nearly homogeneous, compact gray clay, which, possessing the 
property of resisting fire to w marked degree, has from this fact received 
the name it bears. The-fire-clays.are usually penetrated in every direc- 
