12 TREE ANCESTORS 



plant tissues characterize certain horizons in the coal measures 

 especially in England, WestphaUa and Moravia. Silicified plant 

 tissues have rendered the Permian of St. Etienne and Autun in 

 France famous, and quantities of Mesozoic cycads have been simi- 

 larly silicified especially around the rim of the Black Hills. Fossil 

 woods are commonly silicified, opalized or jasperized at very many 

 localities and geological horizons, as in the Arizona National 

 Park, where they are of Permian or Triassic age, or in the Yellow- 

 stone Park where they are Tertiary in age. Scattered trunks and 

 fragments of petrified wood are abundant in all sandy formations. 



The second general method of preservation, by far the most 

 common mode of occurrence of fossil plants, is by simple inclusion 

 in clay, shale, amber or other material, and is often known as 

 incrustation. The bulk of the fossil plants with which the general 

 public is familiar are of this type of preservation which often 

 furnishes the most beautiful impressions of foliage qr even flowers 

 — the finer grained the sediment in which the plant remains were 

 entombed the more perfect the impression. Thus a flocculent 

 calcareous mud or lithographic stone will preserve every detail 

 of form, or a fine mud like that of the fire clays beneath coal beds or 

 the roofing shales above them will be crowded with the delicate 

 remains of the plants that formed the coal, or a fine volcanic dust 

 falling in a lake will preserve most beautiful specimens. Traver- 

 tines will encrust the vegetable debris of deep ravines in a limestone 

 country as in the case of the celebrated early Eocene travertines of 

 Sezanne in France, or the younger travertines of northern Italy, 

 or stalagmite will preserve the litter of burrows or dens of animals 

 or of men of the Old Stone Age that lived in rockshelters. 



Plant remains preserved by inclusion or incrustation may have 

 some of their substance preserved as carbon, it may be replaced 

 by salts of iron or other mineral, or it may be entirely dissipated 

 lea\'ing only the impression or cavity. Lignite beds and coal seams, 

 or the impure peats of buried swamps so common in our Pleisto- 

 cene deposits, are examples of inclusion (incrustation) of plants 

 en masse. Occasionally such swamp deposits of great age like 

 those of Brandon, Vermont, or of the Gennan lignites, both of 



