Devonian Era in the Ohio Basin. — Clay pole. 31 
for the most part the Corniferous limestone is composed, took 
place at the same rate as now, taking the average thickness 
of the stratum at one hundred feet, the length of the Corni- 
ferous period becomes 20,000 years (Dana), or only 5000 
years (LeConte). -A duration of between 10,000 and 20,000 
years may not be far -from the truth. 
During all this time there is no indication of even local 
disturbance on the shores of the Corniferous sea. Except in 
a very few places the purity of the limestones continues to 
the outcrop and it may be legitimately inferred that the period 
was one of rest on land not less than at sea. The southern 
uplift had apparently ceased ; the slight eastern movements 
which had produced the Oriskany sandstone had subsided ; no 
changes are recorded in the north, and the Carboniferous conti- 
nent and whatever other dry areas may have existed cannot 
have presented any very strong relief. They had been eroded 
during the long preceding ages of the Medina, Clinton, Niagara 
and Lower Helderberg periods, so that they were approaching 
a base-level from which little wash was carried down by the 
sluggish streams that drained their gentle slopes. The material 
brought into the sea was chiefly in solution and was separated 
by living organisms for deposit as limestone. 
End of Corniferous Period. — No upper limit has been 
assigned to the Corniferous group. Full consideration of this 
problem cannot well be undertaken until after the next period 
has been discussed. However, what has already been said must 
not be referred to the whole mass of strata which have usually 
been classed under the name Corniferous in Ohio geology, but 
only to that part lying below the Bone-bed and to the Bone-bed 
itself. Where this last exists the physical break is sufficiently 
conspicuous to afford a good and practical base in the field. 
Even where it is absent the difference between the lower solid 
buff limestone and the thinner blue upper beds can be readily 
detected. It is true that this view is opposed to that which was 
maintained by Newberry in the Geology of Ohio, and which 
has been adopted by most subsequent writers. But the evi- 
dence in its favor has so increased since the date of the above 
mentioned volume that at least a reconsideration of the ques- 
tion is justified. This will be done in the succeeding pages. 
At this time the great calcareous floor of the Appalachian gulf 
