feet in thickness, lies 624 feet above sea level and 185 feet 
beneath the surface of the ground which has an elevation 
of 809 feet. 
These coal seams belong to the rocks classed in the 
Des Moines Series of the Upper Pennsylvanian period, 
which is an age equivalent to the Westphalian C, per¬ 
haps to the higher (younger) Westphalian D of western 
Europe (Darrah 1937). It is hoped that a study of the 
plants in this flora will contribute data towards the more 
precise stratigraphy and correlation of the deposits. 
There is one additional feature involved in the occur¬ 
rence of the Iowa coal balls which deserves notice. All of 
the nodules which I have observed in place in the Shuler 
Mine were scattered through the upper third of the coal 
seam, and Mr. Markum called my attention to the fact 
that this is their usual position throughout those por¬ 
tions of the workings where coal balls have been encoun¬ 
tered. The roof of the coal is a black shale which contains 
marine invertebrate fossils. Several years ago my wife 
and I collected many coal balls in the Werister Colliery 
in Liege, Belgium and we observed there that the nod¬ 
ules were scattered through the coal which is overlain 
by a marine band. Many of the Belgian and English 
coal balls contain fossil cephalopods. This fact has given 
rise to the opinion that such nodule formation is “mal 
de mer” of the coal bed. Coal balls are so unexpectedly 
irregular in their occurrence that the intimate association 
of marine sedimentation with coal accumulation usually 
observed in coal ball bearing seams is more than coinci¬ 
dence. 
Acknowledgments 
The extensive collection of coal balls made available 
for investigation were secured by Mr. F. O. Thompson 
with the generous and hearty cooperation of Mr. Henry 
[135] 
