individuals. That this is not merely an apparent rarity is 
indicated by the large number of specimens which have 
been studied. 
Discussion 
The well-known Middle Coal Measures coal ball flora 
of England is a Lyginopteris — Lepidodendron — Cata¬ 
mites flora with Mesoocylon , Medullosa , and filicineans 
being satellitie types. The same association occurs in 
Holland and Belgium, but in the latter Lyginopteris 
seems to be more rare. We have studied nearly a hun¬ 
dred nodules from Belgium and several score from Eng¬ 
land and Holland, and our results on this small number 
agree with those noted in Hirmer’s resumd (1928). 
Judging from the enumerations of species found in 
Illinois coal balls, it would appear that the flora is very 
similar to European coal ball floras except that it includes 
also geologically younger elements, notably the so-called 
Stephanian fern types. 
The Iowa coal ball association is Cordaites — Medul¬ 
losa—Psaronius , with other ferns being complementary 
elements. 
Nature of the coal balls 
Analysis 
The histological detail preserved in these Iowa coal 
balls, although not so excellent as in many types of silic- 
ified nodules, is so good that it is necessary to consider 
the unusual nature of the petrifactions. 
The coal balls are highly pyritic, so much so that the 
specific gravity averages about 3.5 and the luster of a 
cut surface is metallic. In order to verify the optical 
identification of the ingredients in the matrix, a sample 
was submitted to F. A. Gonyer of the Geological Museum 
of Harvard University. He reports the following analysis: 
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