FOSSIL PLANTS 483 



abundance of leaflets of various species of Neuropteris, especially 2f. hirsuta, 

 of Alethopteris Serlii, of Pecopteris villosa, P. abbreviates, Hymenophyllites Clarkii, 

 Annularia longifolia, 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 

 Odontopteris Schlotheimii, Alethopteris erosa, Ulodendron, Oarpolithes 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 Sigillarioides, 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 lamellte 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 



