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 a, marked degree, has from this fact received 

 the name it bearp. The fire-clays are usually penetrated in every direc- 



