482 
PALAEONTOLOGY OF ILLINOIS. 
are flattened for pieces of ferns, in proportion to the breadth of the fragments 
which they have entombed. 
The origin of these concretions has been explained in admitting a general 
tendency of some mineral bodies to concentrate around centers, whether solidify- 
ing from fusion, solution or vapors. (1) This explanation may be satisfactory 
in regard to other kinds of concretions, but from their peculiar position, their 
form and size, varying according to the nature and outline of the bodies which 
they contain, the nodules of Mazon creek rather seem to be the work of infu¬ 
soria or Bacillaria concentrating molecules of iron around some centers, as it 
now happens in the formation of the bog iron ore, or in other deposits, in 
springs or pools, whose waters contain a solution of iron. This supposition ap¬ 
pears confirmed by the manner in which the bodies in concretions have been 
preserved and selected for preservation. Though generally mere fragments, 
their integrity is complete, and yet some of them are of very soft texture. 
The pinnae or leaflets of ferns are always found in them in a flattened position, 
their axis or rachis extending through the center of the elongated nodule, with 
the divisions on both sides; the surface of the pinnules, slightly swollen, as 
when in their living state, is marked by recognizable hairs or fruit dots, with 
distinct veins and veinlets, and their appendages, like the scales, are seen in 
the various modifications which they present in living specimens; for example, 
long, straight, flat, diverging, on primary rachis, and becoming shorter, ruf¬ 
fled and curled on their upper divisions. The small organs of plants appear, 
therefore, in a better state of preservation than in the shales. With small ani¬ 
mals like crustaceans, scorpions, insects of a fleshy and very delicate texture, 
the preservation of form is still more remarkable. They are found entombed 
in the middle of the nodules just as if they were in life, or as if they had been 
transformed into stone while still living. The fruits or nutlets are not flat¬ 
tened. By the section of the nodules, which generally break into two equal 
halves by hard strokes on their edges, the middle and internal part of the fruit 
is exposed to view, while the outside surface is immersed in the stone. The 
numerous cones also of Lepidodendron found in these concretions are equally 
well preserved, either whole or in part, by horizontal cross sections. Some 
specimens not only show distinctly the pedicels of the sporanges and the blades 
in their natural position, but even sporanges with their seeds have been found 
in them, without perceptible alteration. In the cross section of these Lepidos- 
trohi the sporange cells form a central row, which is surrounded by the blades 
in the form of a star. 
Peculiar species of plants and animals, or their fragments, seem to have 
been selected as the nuclei of these nodules. They contain, for example, an 
(1) Dana’s Manual of Geology, p. 626. 
