726 GEOLOGY OF OHIO. 
| Throughout Columbiana county a thick bed of limestone is found al- 
most constantly beneath Coal No. 6, and we learn from Professor Rogers’s 
reports, that the same is true over a large area in western Pennsylvania, 
where the coal is known as the Upper Freeport seam, and the limestone is 
called the Freeport Limestone. 
It must not be inferred from the above facts, that the limestones of 
the Coal Measures are altogether without value for the identification of 
strata, for they are really more persistent and reliable than the shales 
and sandstones with which they are associated. It often happens, how- 
ever, that in tracing a limestone it is found to become earthy and finally 
to pass into a calcareous shale or disappear altogether. In such cases it 
is evident that an unusual quantity of clay was washed into some parts 
of the basin where the limestone was accumulating by organic processes. 
This was sometimes only sufficient in quantity to render the calcareous 
sediment earthy and impure, but sometimes to replace it altogether. 
Another source of error connected with the limestones is that they 
are soluble in atmospheric water, which contains carbonic acid, and 
hence have sometimes disappeared from the outcrops when their as- 
sociated shales and sandstones remain. 
_ The frequent references made in different parts of this Report to the 
Putnam Hill and Zoar limestones of the Tuscarawas Valley, the “‘ White 
Limestone ” of Columbiana county, and the crinoidal limestone of the 
Barren Measures, afford abundant evidence of the value of some of the 
limestones of the coal series as geological guides, and it is probable that 
further exploration will show that the limestones of Jefferson county are 
much more persistent than might be supposed from the observations yet 
made; for since they are generally thin and always readily dissolved, 
their out-crops are undoubtedly often concealed when the strata are 
present. 
Coal No. 7-—This, as has been mentioned, is the highest workable 
seam of the Lower Group. Its geological position is distinctly marked 
by the red and green shales of the Barren Measures which are found 
above it, although sometimes at a considerable interval, and this is the 
first workable seam below them. The Harlem seam referred to on a pre- 
ceding page, lies at least two hundred feet higher, is above the most 
highly colored shales of the Barren Measures and beneath the crinoidal 
imestone. Hence there is little danger of confounding these two coals. 
Coal No. 7 is at Salineville, from 38to 34 feet in thickness, a partially 
open, burning coal, with a moderate percentage of sulphur and ash. 
Passing down the creek from Salineville, Coal No. 7:is seen to thin out 
along the railroad below the station, and it has altogether disappeared 
