46 GEOLOGY OF OHIO. 
We find here the Ferriferous limestone in full force and with all 
the characteristic and valuable qualities of the stratum. The Kittan- 
ning sandstone has become much thicker than it was in our more 
northern sections, and the interval between the limestone and the Lower 
Kittanning coal is increased in this way, but the two Kittanning coals 
(Kittanning and Darlington of the section) serve to hold the section 
steady. Both are universally known and are opened on almost every 
farm. ‘The Kittanning coal is known chiefly in connection with the 
great bed of fire-clay that it covers. It is termed on this account, the 
“Potters’ Vein,” and also by reason of its impure quality, the “Sulphur 
Vein”. The middle Kittanning or Darlington coal is a thin seam, but 
it is everywhere esteemed on account of its excellent quality. It is 
known as the “Block Coal” through the valley. 
In connection with the last section, an observation of White’s is 
well worth repeating here. He says: “The sudden and rapid variations 
which the Ferriferous limestone often undergoes are here well illus- 
trated. At the quarry, we see it 15 feet thick, only a few thin partings 
of shale separating the different layers. As we follow it along the steep 
bluff of the river, all the time perfectly exposed, we see the layers be- 
come arenaceous and shaly. In 400 yards it has entirely disappeared, 
except an impure layer at the top, 6 inches thick, exhibiting the “cone- 
in-cone” structure. ‘There can be no mistake about the change, for the 
exposure is perfect between the two points, and the limestone is seen 
graduating into, and being replaced by sandy shales.” (Q 2, p. 262). 
The sudden disappearance of the Ferriferous limestone has always 
been a source of perplexity to those who have had to do with this stra- 
tum and with its horizon where the limestone has disappeared, and 
while the instructive example just quoted does not enable us to account 
for the surprising change, it is certainly a great advantage to find one 
section adequately laid open in which the change is-accomplished. It 
is more important to learn how the horizon-of the limestone can be fol- 
lowed after the limestone has disappeared. For the sagacious observa- 
tions that have taught us this secret, we are indebted to the work of 
White in Beaver and Lawrence counties. (Reports Q and Q 2.) 
Since leaving the valley of the Middle Fork, we have found but 
little coal associated with the Ferriferous limestone (Upper Clarion), 
while the thin seam first recognized above the Leetonia coal at New 
Lisbon has been developed into a regular and important seam, which 
