414 GEOLOGY OF OHIO. 
tween 10 and 20 feet in thickness. The Ferriferous limestone is fully 
developed in some parts of the field immediately below it, as at Iles- 
boro, Washington township. Northward it is found more or less in all: 
large workings, but it is thin and inconspicuous, seldom showing in 
outcrop, and is often absent from the section altogether. It is often 
replaced or represented by a white flint, as in the southern counties. 
The ore has been mined by stripping only, and the benches of white 
clay, many miles in length, that mark the centers of production, are 
among the most noticeable features of the country. The outcrop ore is 
naturally most valued and sought for, but the seam appears to hold 
quite steady under heavy cover. Its place in the series ranges from 30 
to 50 feet below the Nelsonville coal, which is the standard element in 
all sections in this field. The usual interval between the coal and ore 
is 40 feet. Guided by measurements alone, explorations have revealed 
the presence of the ore in parts of the field where it would scarcely be 
looked for. Mr. W. B. Brooks proved the existence of a fine and 
characteristic stratum of it under his coal at Nelsonville. It is also 
found on the Hayden tract, under quite heavy cover. A thin develop- 
ment of it extends through the hills to the Monday Creek Valley, 
where it has been opened on the Cawthorn farm. It seems to have 
reached its natural limit at this point. It has been mined as far east as 
Shawnee, in Salt Lick township, where it hes just above drainage. 
The ore is almost entirely of the odlitic variety throughout the 
Hocking Valley, and, though not over rich in iron, and quite high in 
silica, it is so free from sulphur and phosphorus, and is otherwise so 
constituted, that it yields an iron of great tenacity and consequent value. 
The mill iron manufactured from it is noted for its strength, and finds 
market even where very rigorous demands as to this quality are made, 
No mention has been made of kidney ores in the field now under 
consideration. There are none that contribute to the iron manufacture 
of the district at the present time. Only one persistent horizon is to 
be found here. The Kittanning shales, occupying in whole or in part 
the interval between the Kittanning coals (Nos. 5 and 6), have been 
found to be ferriferous around the whole margin of the field thus far. 
They offer no exception here. A kidney ore of unusual and marked 
characteristics occurs everywhere in the shales below the Nelsonville 
coal. The kidneys sometimes approach within 2 or 3 feet of the coal, 
