FOSSIL PLANTS 
483 
abundance of leaflets of various species of Neuropteris, especially A". hirsuta, 
of Alethopteris Serlii, of Pecopteris villosa , P. ahbreviata, Hymenophyllites Glarkii, 
Annular ia long (folia, Stigmarioides, etc., which are either rare or have not yet 
been found in the shale at Morris, while these shales are rich in the remains of 
Odontoptexis Schlotheimii , Alethopteris erosa, Ulodendron, Carpolithes multistriatus, 
scarcely or not at all preserved in concretions. As the bank of shale border¬ 
ing the bed of Mazon creek has not yet been opened, these differences may re¬ 
sult from geographical distribution. Yet, as the animals and plants of soft 
exture, like the species of the genus SigiUarioides , have not yet been found in 
the shale of our American Coal Measures, it is evident that these remains have 
been generally destroyed by maceration, and only escaped total destruction by 
their entombment in these nodules. The same can be remarked on the re¬ 
mains of small animals. The remains of fishes found in these concretions are 
merely bones, scales and coprolites, while of molluscs, they have afforded only 
some agglomerations or very small shells. 
§ 4. VEGETABLE REMAINS PRESERVED BY MINERALIZA¬ 
TION OR TRUE PETRIFICATION. 
This kind of fossilization is performed by slow infiltration of mineral matter 
into the substance of the vegetable, when in a soft state of decomposition. The 
phenomenon is produced either by a total destruction of the vegetable substance, 
for which sand, clay or oxyd of iron is substituted by infiltration, or by a slow, 
still unexplained mineralization of the vegetable substance, by silex or lime. 
By the first process, the whole texture of the vegetable is destroyed, except 
the surface, preserved as in a mould, which shows the original outline of the 
vegetable, and bears the cicatrices of the bark and other external characters, 
which often render it recognizable. These moulds, generally covered by a 
coat of coaly matter, are rarely flattened by compression, and mostly represent 
trunks or branches of large size, sometimes fruits of a hard consistence, rarely 
branches and leaves of ferns. They abound in the sandstone beds of our 
Coal Measures, and some of our new species of Lepidodendron and of Sigillaria, 
have been described from specimens of this kind. In the second case of petri¬ 
fication, on the contrary, the surface or outside of the vegetables is generally 
obliterated, as if it had been more or less decayed while subjected to minerali¬ 
zation, while the internal structure is preserved in its minutest details, and so 
distinctly, that it can be studied under the microscope when lamelUe of the 
fossils are detached, and polished thin enough to become transparent. Speci¬ 
mens of wood fossilized in this way, though often remarked in the Carbonifer¬ 
ous formations of Europe, and very common in the more recent formations of 
