344 FOSSIL PLANTS. 
whole mass, the remains of plants, following the force of upheaval, 
have been, at peculiar places, drawn upwards and therefore elon- 
gated on one side, and of course drawn on the other towards the 
rachis. It is peculiar that the rachis and stems do not show any 4 
appearance of flexure and of deformation, and it is remarkable _ 
also that the same phenomenon of dimorphism is not observable _ 
on the plants found in the shale of the anthracite basin of Penn- 
sylvania, where the flexures of the veins of coal are often abrupt, 
and where traces of tortion are frequently seen upon fragments of 
the combustible mineral. This deformation of vegetable remains 
may give an idea of the difficulties encountered by the palæontolo- 
gist in studying, as he has to do, mere fragments of plants in their 
fossil state. Not only do these remains generally insufficiently 
represent the whole vegetable, but often they are deformed by 
various forces and influences, to which they are subjected in the 
process of mineralization. E 
VEGETABLE REMAINS PRESERVED IN FerruGIxous Concretions. 
—As far as we know, from the specimens abundantly found in 
Illinois, the mode of preservation of fossil plants in concretions 
is somewhat different from what it is in argillaceous shale. These 
concretions are found, especially in the shale of Grundy county, 
irregularly scattered from top to bottom of the strata, in the form 
of oval, more or less elongated, generally slightly flattened cons 
cretions. They appear to have been formed by superposition of 
concentric layers of slowly deposited carbonate of iron or ferrugi- 
nous clay around central nuclei, which are most commonly parts 
of plants, bones of fishes or the remains of insects and crustacea: 
Their size and form vary according to that of the body around 
which the deposit has been made. Some small leaflets of ferns are 
found 
