1036 GEOLOGY OF OHIO. 
sented in vertical sections. In the map of Lawrence and Scioto coun- 
ties, which accompanies this volume, the areas occupied by the limestone 
above drainage are shown, a task which would be impossible without 
the expenditure of great labor for any other elements of the scale 
except those directly connected with it. The limestone has been worked 
along its outcrops in practically continuous benchings throughout most 
of the furnace properties and through a great deal of other territory. 
The Ferriferous horizon indicates as well the horizon of the Kittanning 
coals, the lower of these seams being but 20 or 30 feet above the lime- 
stone, and the upper only 60 or 70 feet above, so that this boundary in 
Lawrence and Scioto and in Jackson and Vinton agrees very closely 
with the Kittanning boundary, as laid down upon the maps of the 
counties to the northward. Upon maps of the scale here employed, the 
two boundaries are, for much of the territory, practically identical. 
Another ore horizon is very widely known and used, especially in 
determining the order of the lower portions of the series. It is the 
horizon of the main or upper block ore of the Ohio Valley. It is 
well named by Mr. Brown, the Franklin block ore, it having been the © 
chief dependence of Franklin Furnace when in blast. It lies 110 to 
120 feet below the Ferriferous, and 40 to 70 feet above the Lower 
Mercer limestone.. There is pretty good reason to believe that this ore 
belongs to the Upper Mercer limestone, but whatever its exact northern 
continuation may be found to. be, it is one of the steadiest and most 
easily recognized beds of the entire series. in Southern Ohio. Mr. 
Brown made constant use of it in the western part of this field, and 
found it a guide almost as serviceable as the Ferriferous limestone ore. 
At many of the furnaces it is known as the big red block. 
The little red block ore, which is carried by the Lower. Mercer 
limestone, helps to support and confirm the sections. In determining 
the upper coals, the Cambridge limestone becomes of great value. It 
is constant in occurrence, and unmistakable in character, and leaves 
nothing to be desired as a ruling element in a geological section. | 
By the aid of these three well-known elements, all of the coal 
seams of the district can be so located that little trouble will:be ex- 
perienced in assigning any exposure to its proper place in the scale. 
The coal seams that are due in this field are the 12 seams of the 
Lower Coal Measures (see page 127), but several of the series have not 
thus far been found of economic importance within the district. The 
