FOSSIL PLANTS. 479 



fused to cohesion after the transformation of vegetable matter into coal, one 

 can easily discover an abundance of remains of plants whose genera and even 

 species are sometimes recognizable. These facts, which cannot be overlooked, 

 may be taken into account in examining new theories in relation to the forma- 

 tion of coal. 



2. VEGETABLE REMAINS PRESERVED IN SHALE. 



It is in the clay or silicious shale that the fragments of plants of the coal 

 epoch have been more generally preserved. When a bed of vegetable matter 

 heaped for the formation of a coal has begun to cease its growth, its top indi- 

 cates a greater scarcity of vegetable remains, mixed with a larger proportion of 

 earthy or clayey matter. The coal then becomes a less homogeneous mass, 

 easily separating in layers of heaped fragments of vegetable and foreign mat- 

 ter. By and by, the vegetation becoming scarcer by superabundance of water 

 upon the surface of the bogs, the clay is more thickly deposited, and the vege- 

 table remains, more rare and scattered, are more distinct and more easily re- 

 cognizable. When preserved in that way, the plants or their fragments have 

 been first slowly decomposed 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 vegetable and is 

 marked by the impression of veins and veinlets, mostly distinct to their last 

 divisions. Some leaves of a coriaceous texture have their epidermis hard- 

 ened by mineralization, and separable from the shale like a transparent pelli- 

 cle. 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 clcgans, 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 cuticle, preserving traces of the primitive or- 

 ganism. 



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 



