446 GEOLOGY OF OHIO. 
The rocks of the coal strata in Ohio lie nearly horizontal; they are 
subject to no great uplifts or faults, and while the dip varies consider- 
ably, it rarely exceeds 30 feet per mile in a direction south of east. 
The axis or central line of the whole Allegheny basin is found to pass 
near Wheeling, in West Virginia, in a northeasterly and southwesterly 
direction, and in the vicinity of Wheeling we also find the greatest 
vertical thickness of the coal Measures, which is abont 1,500 feet. 
The strata of the Coal Measures are the most recent of the con- 
solidated rocks of Ohio, and never were submerged after the time of 
their formation long enough to receive the deposition of any subsequent 
strata. ‘They have hence been subjected for untold ages to the power- 
ful denuding action of the atmosphere and water. Ceaseless erosion 
has probably removed the coal rocks from considerable areas which 
they once occupied, and has deeply furrowed the present area with in- 
numerable valleys, which at one place or another expose to view the 
entire series of the Coal Measures. These valleys are often 400 feet 
below the summits of the adjoining hills. Their excavation has brought 
the deepest coals of the series within 800 feet of the surface, and ex- 
posed the various coals and iron ores on their slopes, in position which 
render their accessibility for ease and cheapness of exploitation un- 
surpassed. The exposures are so frequent and numerous that there is 
hardly a township in the entire coal area where coal is not drifted for, 
to supply the wants of the inhabitants ; indeed, it might almost be said 
that each farmer has his own coal bank. 
The total thickness of the coal bearing rocks in Ohio, as already 
stated, is about 1,500 feet, though over the greater part of the area the 
thickness is hardly more than one-half of this. The total] thickness is 
divided by some 400 feet of barren shales (the Barren Coal Measures) 
into the coal groups of the Lower and Upper Coal Measures, which 
distinction holds with great completness over the whole area of the 
Allegheny coal field. The labors of the Geological Survey have re- 
corded eleven workable seams in the Lower group—one at the base of the 
Barren Coal Measures, and six in the Upper group, or eighteen work- 
able seams in all. These coals vary from 3 to 6 or 7 feet, and some- 
times 10 feet in thickness, no seam being considered as generally work- 
able where the thickness is below 3 feet. Beside these eighteen prin- 
cipal seams of coal, there are others intercalated, which, however, are 
generally of only local extent and importance. These coals have been 
