J. Hall on Carboniferous Limestones of Mississippi Valley. 197 
A still more interesting exhibition of phenomena attendant 
upon this condition of the strata, and consequent upon this 
ancient denudation, is the occurrence, in the limestones of the 
age of the Hamilton and Upper Helderberg groups, of rounded 
or irregular masses of clay, like the underclay of coal seams. 
These masses which are seen in sections along the river, and in 
quarries, often present simply the appearance of a spheroidal 
re 
fin 
former adhering closel , and when separated, the limestone still 
asses of clay were of subsequent deposition to the limestone, 
and that they filled cavities which had been made by denudation 
like modern caverns in limestone. This example was seen in 
the vertical 
4 F 
ce of a quarry presenting an elevation of 80 or 
he 3. 
=3. SS : eran Serer AS 
SNES eae Psa FR owe 5 
erry Sy ak i st RS 
EE ew 
sok 
aka! 
Rees 
x 
i a, a, a, Limestone of Devonian age. : ee 
toe lal ikea eppeet ear care ee 
+, _Jhis-cavity from top to bottom is filled with hard lay like 
the underclay of coal seams. At the mouth of the fu it is 
of a reddish brown hue, but soon becomes of the ordinary gray 
Aue 
