378 GEOLOGY OF OHIO. 
separate and distinct from it, but lying in close proximity. Above the 
main sheet of ore, kidneys are generally to be found scattered through 
the clay. A common and very characteristic form of the unaltered ore 
of this horizon is found in the so-called gray ore. It consists of minute 
grains of carbonate of iron, buried in a silicious clay. This form does 
not blend with the limestone. The most valued form of the ore, 
especially for use in charcoal furnaces, is the limonite that has resulted 
from the weathering and transformation of the original carbonate. 
A second phase of limestone ore, but not usually recognized by 
that name, is the replacement of the buff limestones of the Freeport 
type with carbonate of iron to a greater or less extent. These ores are 
very uncertain in character, changing from ore to limestone on short 
notice. On their outcrops they are frequently ores of fair grade, while 
under cover they are simply ferruginous limestones. The impure lime- 
stones of this group are frequently nodular, lying in detached masses 
in their clays, and when these are transformed into limonites as far as 
their iron will allow, they pass for kidney ores, but they do not show 
the same history as the ores to which this name has already been 
assigned. 
The ores of the Lower Coal Measures will all be included under 
one or another of the forms now described, but distinct attention must 
be called to a line of facts which has several times, in the course of this 
discussion, been mentioned incidentally. These ores, while originally 
carbonates of iron in every case, have been transformed along their 
lines of outcrop, and often under considerable cover, into hydrated 
peroxides or limonites. In many instances the transformation has been 
very thorough, the form, volume, specific gravity, texture, and color of 
the ore being changed in the process. The change is always in the line 
of improvement of the quality of the ore. 
GEOLOGICAL ORDER OF THE IRON ORES OF OHIO. 
I. THE STRATIFIED ORES. 
The ores of this list, consisting mainly of blackband, but also 
including some clayband and some undescribed forms known by local 
names, belong to the three distinct horizons named below: 
