The small crystals of pyrite have not distorted the 
plant tissues during petrifaction. Study of a ground sec¬ 
tion of a coal ball reveals that the pyrite is distributed 
through the matrix in the spaces in and between plant 
fragments, not in the cells or tissues. 
Almost nothing is known concerning the mode of 
formation of such nodules. The sulphur content, which is 
in the form of one or more sulphides, is proportionately 
higher than in the surrounding coal, and it is possible that 
this indicates that some organic factor associated with 
the complex process of decomposition is influential in 
their formation. Small masses of pyrite or marcasite are 
to be observed in nearly every coal seam, and I suspect 
that they owe their origin to a kindred, if not identical, 
process. Stopes and Watson (1908) and Feliciano (1924) 
agree that coal balls are formed in situ and found no 
evidence in support of the naive hypothesis that the nod¬ 
ules were transported into a coal-forming environment. 
Occurrence 
The coal balls reported in this paper have been found 
in a small area west of the city of Des Moines, Iowa, 
traversing the boundary between Dallas and Polk Coun¬ 
ties. The most abundant material has been collected in 
the Shuler Mine in Walnut Township, Dallas County, 
five miles west of Des Moines. The Urbandale Mine, 
Walnut Township, two miles west of the city limits of 
Des Moines, has also yielded abundant material. 
It is not known whether the coal seams of the Shuler 
and Urbandale Mines are identical. Coal number 7 of the 
Shuler Mine is a seam five and one half feet thick, lying 
610 feet above sea level and 387 feet beneath the surface 
of the ground which is at an elevation of 997 feet. The 
chief seam of the Urbandale Mine, that which contains 
nodules, varies from four and one half to five and one half 
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