134 GEOLOGY OF OHIO. 
the several coal seams already named will also be characterized in gen- 
eral terms before the detailed examination of the various coal-producing 
districts of the State is entered upon. 
THE Onto CoAL FIELD. 
The Ohio coal field is the northwestern prolongation of the Appa- 
lachian coal field, which, with its outliers, constitutes by far the most 
important source of coal for the North American Continent. The area 
of the entire field is estimated approximately at 60,000 square miles. 
(59,105 square miles, C. H. Hitchcock, Ninth Census, U.S.) One-sixth 
of this area is estimated to belong to Ohio. (10,000 square miles, New- 
berry.) 
The whole field is one of remarkable symmetry and regularity, and 
of its several subdivisions, the portion belonging to Ohio and North- 
western Pennsylvania is the most symmetrical and regular. There is 
nowhere known a more orderly series of coal measure deposits than 
those included in this territory. 
It is traversed by a few gentle folds, but these serve to aid rather 
than to confuse the reading of the system. There is not throughout the 
Ohio field a fault worthy of the name, and there is no unconformability, 
except the slight amount due to overlap in the lowest portion of the 
series. The dip of the strata is so slight that it can be determined only 
by triangulation. The clinometer is a superfluous instrument in Ohio. 
The physical side of the history of the growth of the Coal Meas- 
ures of Ohio is strictly in keeping with the same phase of the history 
of the underlying formations for several previous geological periods. 
At the beginning of Lower Silurian time, all of Ohio lay beneath 
that arm of the sea which was enclosed between the Appalachian border 
and the Canadian nucleus of the continent, and the dwarfed and 
shrunken representative of which we find in the northern portion of the 
Gulf of Mexico to-day. 
At the end of the Lower Silurian age, a low fold had entered the 
State at its southwestern corner, advancing from Tennessee and Ken- 
tucky to the northeastward. It is known as the Cincinnati Axis. 
The gradual growth and extension of this axis are facts of funda- 
mental importance in the subsequent geological history of the State. 
It was advancing slowly through Upper Silurian and early Devonian 
