486 GEOLOGY OF OHIO. 
but the oxidizing action of water and atmosphere has changed them, 
wherever such action was possible, into hydrous peroxides or limonites. 
The mining only exceptionally being more than stripping off the out- 
crop, the part of the beds beyond the action of oxidation where the iron 
is as a carbonated base furnishes but a small proportion of the ores ob- 
tained. 
There are four perfectly distinct and persistent seams of ore, varying 
from 4 to 16 inches in thickness, beside several accumulations of kidney 
ore in the accompanying beds of shale. 
The main beds of ore are the “ Limestone ore,” the “ Big Red Block 
ore,” the “Sand Block ore,” and the “ Little Red Block ore.”” Also in the 
shales above the limestone ore, nodules or kidneys of argillaceous iron 
ore are found, which often become very abundant. At different 
horizons in the region are found other strata of shale which sometimes 
contain quite important amounts of this kidney, generally, however, at 
a higher level, and commonly known as “top hill ores.” These 
nodules of kidney ore are never of sufficient abundance to warrant 
regular mining operations. 
The limestone ore is by far the most important one of the series, both 
in richness and quantity, and the one from which two-thirds of the iron is 
made in the region. Its name arises from its position, overlying a lime- 
stone which is one of the most well-marked and persistent features in 
the geology of the region. This limestone, used constantly as a base line 
in surveying, furnishes the flux for the furnaces of the region, and is 
traceable across the Ohio into Kentucky, and northward into Northern 
Ohio. ‘The limestone ore is found in its original state of a carbonate 
of iron, but mining operations along its outcrop have developed it 
mostly as a dark red hydrous peroxide or limonite, lying on the lime- 
stone in a regular bed, averaging about 12 inches thick, although in 
‘one or two instances basins have been found 5 feet in thickness. The 
“red limestone” is the richest and most valued ore, and contains on an 
average about 40 per cent. of metallic iron. It is the most important 
and widely used ore in the region, and the one more than any other 
upon which the high reputation of the Hanging Rock irons has been 
dependent. . Without the “red limestone” ore in some proportions, 
there is little iron made in the region, and those furnaces producing the 
best quality of pig-iron use it almost exclusively. The ore varies in 
character from a hard compact limonite ore to one soft and ochreous, and 
