Fossil Botany. 23 



have before stated, identical recent species have not been dis- 

 covered. When these nodules are carefully broken, the im- 

 pressions are seen preserved on both faces of the clay,— but 

 contrary to expectation, not displaying each side of the leaf or 

 stem, but the same side on each broken surface ; on the one m 

 alto, and on the other in basso relievo, or slightly depressed on 

 one side and slightly raised on the other. The best explana- 

 tion of this curious circumstance, which long puzzled observ- 

 ers, appears to be the following.— The vegetable matter, in 

 passing through its bituminous change, first became softened, 

 and filled its own mould between^the walls of clay with the 

 liquid or deliquescent substance thus produced; this subse- 

 quently became hardened, and adhered to one side of the clay, 

 which on being broken shows the surface of the adherent 

 bituminous cast, while the other side displays the corresponding 

 mould. The adjoining cut will give an idea of such appear- 

 ances. It represents a species of fern or Polypodium, in clay 

 slate, from an English coal mine. Sir James E. Smith considers 

 it the product of a tropical climate, but not referable to any 

 recent species. 



It is well known to geologists that nearly all the plants, par- 

 ticularly those of the cryptogamous tribes, as the Ferns, and 

 Algae, or sea weeds, now found in geological formations in all 

 parts of the earth, indicate by their increased proportions, the 

 influence of tropical regions. Moisture and heat appear to be 

 the conditions of the largest developement of these plants. 

 Thus, the Ferns, as the brake, Pteris, or polypody, Polypodium, 



