342 FOSSIL PLANTS. 
way, the plants or their fragments have been first slowly decom- 
posed and softened by humidity, and then more or less flattened 
by compression. All the naturalists who have examined the coal 
formations are well acquainted with the appearance of the remains 
found in shale, and sometimes admirably preserved. Generally, 
the woody tissue of the plant has been destroyed, and the surface 
of the stems and branches only are preserved in a thin coat of 
coaly matter, bearing impressions of scars of the bark, etc. For 
the leaves, the coaly matter represents the whole substance, and 
for the ferns, especially, it preserves the exact form of the vege- 
table, and is marked by the impression of veins and veinlets, mostly 
distinct to their last divisions. Some leaves of a coriaceous text- 
ure have their epidermis hardened by mineralization, and separa- 
ble from the shale like a transparent pellicle. It can then be easily 
examined under the microscope, and all the details of structure 
recognized. It is especially the case with our Dictyopteris rubella 
of Murphysborough, as also with the leaves of Whittleseya elegans 
Newb., of Ohio. Sometimes the leaves of Neuropteris hirsuta 
have been heaped and compressed together in such quantity, that 
the pinnules are separable from each other as a carbonaceous cuti- 
cle, preserving traces of the primitive organism. 
The shales, according to the amount of vegetable matter mixed 
in them, and the depth at which they have been formed under 
water, are of a more or less dark color; whitish or yellowish when 
of fresh water origin, and with few remains of plants; black and 
generally more homogeneous when formed in deep water, and hay- 
ing for a larger proportion of their compound, broken remains of 
organized beings. In this case the remains are either animal or 
vegetable mixed together, both fragments of molluscs and fishes 
with fragments of plants recognizable on the same piece of shale, 
or mere remains of animals, or only plants. These various appear- 
ances are easily explained in considering the phenomena accom- 
panying the formation of the coal strata, from deposits analogous 
to those of our existing peat bogs. For the surface of these bogs, 
even in our time, shows the same differences in the superposed de- 
posits, according to the depth and chemical compounds of the 
water by which they become covered, either by casual inundation 
in the interior of the land, or by slow immersion near the borders 
of lakes or sea shores. Even where the coal and shales, from the 
amount of remains of fishes which they contain, appear to have 
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