FOSSIL PLANTS. 345 
nutlets for nuclei, are round or exactly oval, while they are flat- 
tened for pieces of ferns, in proportion to the breadth of the frag- 
ments which they have entombed 
The origin of these concretions has been explained in admit- 
ting a general tendency of some mineral bodies to concentrate 
around centres, whether solidifying from fusion, solution, or vapors.* 
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 infusoria or Bacillaria concentrating molecules of iron 
around some centres, 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 appears con- 
firmed by the manner in which the bodies in concretions have been 
preserved and selected for preservation. ough generally mere 
fragments, their integrity is complete, and yet some of them are of 
very soft texture. The pinne or leaflets of ferns are always found 
in them in a flattened positi their axis or rachis extending 
through the centre 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, ruffied and curled on their 
upper divisions. e 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 deli- 
cate 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 flattened. By the 
section of the nodules, which generally break into two equal halves 
by hard strokes upon 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 
*Dana’s Manual of Geology, p. 626. 
