Devonian Era in the Ohio Basin. — Claypole. 87 
to at least the Cretaceous basin, was an old land-surface where- 
on no later deposits have ever apparently existed than the Ozark 
series. It is consequently in this gap between the Cincinnati 
promontory and the Ozark mountains that an opening to the 
southward must be found, if it be found at all. 
Beginning at the base of the Silurian, the Clinton deposits 
skirt the western side of the promontory, appearing near the 
falls of the Ohio. To the westward little or nothing is known 
of them. Their characters are lost in those of the great mass 
of limestone above them, so that they cannot be certainly distin- 
guished from the Niagara. Or, possibly, they are not present 
at all. From the statement that, in southern Illinois, the Niag- 
ara limestone is seen resting directly on the Cincinnati group :;: 
we may confidently infer that in that quarter the latter suppo- 
sition is correct. 
Farther to the westward, in Missouri, no record of the Clin- 
ton exists, but as traces of a land area are lacking it may be saf- 
er to infer that it is merged in the Niagara and at present indis- 
tinguishable from that group. In the next period the evidence is 
more satisfactory. Ranging southwest from the falls of the 
Ohio, the outcrop of the Niagara sweeps into Illinois and Mis- 
souri, where it is found both north and south of the ridge of 
Ordovician projecting eastward from the Ozarks. In the south- 
ern area it thickens considerably and becomes argillaceous, in- 
dicating proximity to land. But farther south it disappears, 
and beyond Cape Girardeau count}' the limestone of that name 
(Ordovician) comes to the surface. 
The story of the Lower Helderberg is somewhat different. 
Also skirting the western side of the promontory it overlaps 
the older Silurian strata and sweeps far south through Ken- 
tucky and Tennessee to the north line of Alabama and Missis- 
sippi, where it is lost. To find it again we must pass into Mis- 
souri, where it reappears with tbe Niagara and is 200 feet thick 
in Cape Girardeau count;.', leaving an interval in the latitude <>| 
northern Tennessee of less than 100 miles, in the Mississippi 
basin, altogether unknown. 
All attainable evidence tends to show that the Silurian sea^ 
never extended over the Missouri-Arkansas state-line. More- 
over on both sides of the unknown region in Missouri ami in 
* Geol. of III., vol. iv, 1). 140. 
